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diff --git a/doc/src/sgml/spgist.sgml b/doc/src/sgml/spgist.sgml new file mode 100644 index 0000000..102f862 --- /dev/null +++ b/doc/src/sgml/spgist.sgml @@ -0,0 +1,1076 @@ +<!-- doc/src/sgml/spgist.sgml --> + +<chapter id="spgist"> +<title>SP-GiST Indexes</title> + + <indexterm> + <primary>index</primary> + <secondary>SP-GiST</secondary> + </indexterm> + +<sect1 id="spgist-intro"> + <title>Introduction</title> + + <para> + <acronym>SP-GiST</acronym> is an abbreviation for space-partitioned + <acronym>GiST</acronym>. <acronym>SP-GiST</acronym> supports partitioned + search trees, which facilitate development of a wide range of different + non-balanced data structures, such as quad-trees, k-d trees, and radix + trees (tries). The common feature of these structures is that they + repeatedly divide the search space into partitions that need not be + of equal size. Searches that are well matched to the partitioning rule + can be very fast. + </para> + + <para> + These popular data structures were originally developed for in-memory + usage. In main memory, they are usually designed as a set of dynamically + allocated nodes linked by pointers. This is not suitable for direct + storing on disk, since these chains of pointers can be rather long which + would require too many disk accesses. In contrast, disk-based data + structures should have a high fanout to minimize I/O. The challenge + addressed by <acronym>SP-GiST</acronym> is to map search tree nodes to + disk pages in such a way that a search need access only a few disk pages, + even if it traverses many nodes. + </para> + + <para> + Like <acronym>GiST</acronym>, <acronym>SP-GiST</acronym> is meant to allow + the development of custom data types with the appropriate access methods, + by an expert in the domain of the data type, rather than a database expert. + </para> + + <para> + Some of the information here is derived from Purdue University's + SP-GiST Indexing Project + <ulink url="https://www.cs.purdue.edu/spgist/">web site</ulink>. + The <acronym>SP-GiST</acronym> implementation in + <productname>PostgreSQL</productname> is primarily maintained by Teodor + Sigaev and Oleg Bartunov, and there is more information on their + <!-- URL will be changed --> + <ulink url="http://www.sai.msu.su/~megera/wiki/spgist_dev">web site</ulink>. + </para> + +</sect1> + +<sect1 id="spgist-builtin-opclasses"> + <title>Built-in Operator Classes</title> + + <para> + The core <productname>PostgreSQL</productname> distribution + includes the <acronym>SP-GiST</acronym> operator classes shown in + <xref linkend="spgist-builtin-opclasses-table"/>. + </para> + + <table id="spgist-builtin-opclasses-table"> + <title>Built-in <acronym>SP-GiST</acronym> Operator Classes</title> + <tgroup cols="3"> + <thead> + <row> + <entry>Name</entry> + <entry>Indexable Operators</entry> + <entry>Ordering Operators</entry> + </row> + </thead> + <tbody> + <row> + <entry valign="middle" morerows="11"><literal>box_ops</literal></entry> + <entry><literal><< (box,box)</literal></entry> + <entry valign="middle" morerows="11"><literal><-> (box,point)</literal></entry> + </row> + <row><entry><literal>&< (box,box)</literal></entry></row> + <row><entry><literal>&> (box,box)</literal></entry></row> + <row><entry><literal>>> (box,box)</literal></entry></row> + <row><entry><literal><@ (box,box)</literal></entry></row> + <row><entry><literal>@> (box,box)</literal></entry></row> + <row><entry><literal>~= (box,box)</literal></entry></row> + <row><entry><literal>&& (box,box)</literal></entry></row> + <row><entry><literal><<| (box,box)</literal></entry></row> + <row><entry><literal>&<| (box,box)</literal></entry></row> + <row><entry><literal>|&> (box,box)</literal></entry></row> + <row><entry><literal>|>> (box,box)</literal></entry></row> + + <row> + <entry valign="middle" morerows="10"><literal>inet_ops</literal></entry> + <entry><literal><< (inet,inet)</literal></entry> + <entry valign="middle" morerows="10"></entry> + </row> + <row><entry><literal><<= (inet,inet)</literal></entry></row> + <row><entry><literal>>> (inet,inet)</literal></entry></row> + <row><entry><literal>>>= (inet,inet)</literal></entry></row> + <row><entry><literal>= (inet,inet)</literal></entry></row> + <row><entry><literal><> (inet,inet)</literal></entry></row> + <row><entry><literal>< (inet,inet)</literal></entry></row> + <row><entry><literal><= (inet,inet)</literal></entry></row> + <row><entry><literal>> (inet,inet)</literal></entry></row> + <row><entry><literal>>= (inet,inet)</literal></entry></row> + <row><entry><literal>&& (inet,inet)</literal></entry></row> + + <row> + <entry valign="middle" morerows="5"><literal>kd_point_ops</literal></entry> + <entry><literal>|>> (point,point)</literal></entry> + <entry valign="middle" morerows="5"><literal><-> (point,point)</literal></entry> + </row> + <row><entry><literal><< (point,point)</literal></entry></row> + <row><entry><literal>>> (point,point)</literal></entry></row> + <row><entry><literal><<| (point,point)</literal></entry></row> + <row><entry><literal>~= (point,point)</literal></entry></row> + <row><entry><literal><@ (point,box)</literal></entry></row> + + <row> + <entry valign="middle" morerows="11"><literal>poly_ops</literal></entry> + <entry><literal><< (polygon,polygon)</literal></entry> + <entry valign="middle" morerows="11"><literal><-> (polygon,point)</literal></entry> + </row> + <row><entry><literal>&< (polygon,polygon)</literal></entry></row> + <row><entry><literal>&> (polygon,polygon)</literal></entry></row> + <row><entry><literal>>> (polygon,polygon)</literal></entry></row> + <row><entry><literal><@ (polygon,polygon)</literal></entry></row> + <row><entry><literal>@> (polygon,polygon)</literal></entry></row> + <row><entry><literal>~= (polygon,polygon)</literal></entry></row> + <row><entry><literal>&& (polygon,polygon)</literal></entry></row> + <row><entry><literal><<| (polygon,polygon)</literal></entry></row> + <row><entry><literal>&<| (polygon,polygon)</literal></entry></row> + <row><entry><literal>|>> (polygon,polygon)</literal></entry></row> + <row><entry><literal>|&> (polygon,polygon)</literal></entry></row> + + <row> + <entry valign="middle" morerows="5"><literal>quad_point_ops</literal></entry> + <entry><literal>|>> (point,point)</literal></entry> + <entry valign="middle" morerows="5"><literal><-> (point,point)</literal></entry> + </row> + <row><entry><literal><< (point,point)</literal></entry></row> + <row><entry><literal>>> (point,point)</literal></entry></row> + <row><entry><literal><<| (point,point)</literal></entry></row> + <row><entry><literal>~= (point,point)</literal></entry></row> + <row><entry><literal><@ (point,box)</literal></entry></row> + + <row> + <entry valign="middle" morerows="9"><literal>range_ops</literal></entry> + <entry><literal>= (anyrange,anyrange)</literal></entry> + <entry valign="middle" morerows="9"></entry> + </row> + <row><entry><literal>&& (anyrange,anyrange)</literal></entry></row> + <row><entry><literal>@> (anyrange,anyelement)</literal></entry></row> + <row><entry><literal>@> (anyrange,anyrange)</literal></entry></row> + <row><entry><literal><@ (anyrange,anyrange)</literal></entry></row> + <row><entry><literal><< (anyrange,anyrange)</literal></entry></row> + <row><entry><literal>>> (anyrange,anyrange)</literal></entry></row> + <row><entry><literal>&< (anyrange,anyrange)</literal></entry></row> + <row><entry><literal>&> (anyrange,anyrange)</literal></entry></row> + <row><entry><literal>-|- (anyrange,anyrange)</literal></entry></row> + + <row> + <entry valign="middle" morerows="9"><literal>text_ops</literal></entry> + <entry><literal>= (text,text)</literal></entry> + <entry valign="middle" morerows="9"></entry> + </row> + <row><entry><literal>< (text,text)</literal></entry></row> + <row><entry><literal><= (text,text)</literal></entry></row> + <row><entry><literal>> (text,text)</literal></entry></row> + <row><entry><literal>>= (text,text)</literal></entry></row> + <row><entry><literal>~<~ (text,text)</literal></entry></row> + <row><entry><literal>~<=~ (text,text)</literal></entry></row> + <row><entry><literal>~>=~ (text,text)</literal></entry></row> + <row><entry><literal>~>~ (text,text)</literal></entry></row> + <row><entry><literal>^@ (text,text)</literal></entry></row> + </tbody> + </tgroup> + </table> + + <para> + Of the two operator classes for type <type>point</type>, + <literal>quad_point_ops</literal> is the default. <literal>kd_point_ops</literal> + supports the same operators but uses a different index data structure that + may offer better performance in some applications. + </para> + <para> + The <literal>quad_point_ops</literal>, <literal>kd_point_ops</literal> and + <literal>poly_ops</literal> operator classes support the <literal><-></literal> + ordering operator, which enables the k-nearest neighbor (<literal>k-NN</literal>) + search over indexed point or polygon data sets. + </para> + +</sect1> + +<sect1 id="spgist-extensibility"> + <title>Extensibility</title> + + <para> + <acronym>SP-GiST</acronym> offers an interface with a high level of + abstraction, requiring the access method developer to implement only + methods specific to a given data type. The <acronym>SP-GiST</acronym> core + is responsible for efficient disk mapping and searching the tree structure. + It also takes care of concurrency and logging considerations. + </para> + + <para> + Leaf tuples of an <acronym>SP-GiST</acronym> tree usually contain values + of the same data type as the indexed column, although it is also possible + for them to contain lossy representations of the indexed column. + Leaf tuples stored at the root level will directly represent + the original indexed data value, but leaf tuples at lower + levels might contain only a partial value, such as a suffix. + In that case the operator class support functions must be able to + reconstruct the original value using information accumulated from the + inner tuples that are passed through to reach the leaf level. + </para> + + <para> + When an <acronym>SP-GiST</acronym> index is created with + <literal>INCLUDE</literal> columns, the values of those columns are also + stored in leaf tuples. The <literal>INCLUDE</literal> columns are of no + concern to the <acronym>SP-GiST</acronym> operator class, so they are + not discussed further here. + </para> + + <para> + Inner tuples are more complex, since they are branching points in the + search tree. Each inner tuple contains a set of one or more + <firstterm>nodes</firstterm>, which represent groups of similar leaf values. + A node contains a downlink that leads either to another, lower-level inner + tuple, or to a short list of leaf tuples that all lie on the same index page. + Each node normally has a <firstterm>label</firstterm> that describes it; for example, + in a radix tree the node label could be the next character of the string + value. (Alternatively, an operator class can omit the node labels, if it + works with a fixed set of nodes for all inner tuples; + see <xref linkend="spgist-null-labels"/>.) + Optionally, an inner tuple can have a <firstterm>prefix</firstterm> value + that describes all its members. In a radix tree this could be the common + prefix of the represented strings. The prefix value is not necessarily + really a prefix, but can be any data needed by the operator class; + for example, in a quad-tree it can store the central point that the four + quadrants are measured with respect to. A quad-tree inner tuple would + then also contain four nodes corresponding to the quadrants around this + central point. + </para> + + <para> + Some tree algorithms require knowledge of level (or depth) of the current + tuple, so the <acronym>SP-GiST</acronym> core provides the possibility for + operator classes to manage level counting while descending the tree. + There is also support for incrementally reconstructing the represented + value when that is needed, and for passing down additional data (called + <firstterm>traverse values</firstterm>) during a tree descent. + </para> + + <note> + <para> + The <acronym>SP-GiST</acronym> core code takes care of null entries. + Although <acronym>SP-GiST</acronym> indexes do store entries for nulls + in indexed columns, this is hidden from the index operator class code: + no null index entries or search conditions will ever be passed to the + operator class methods. (It is assumed that <acronym>SP-GiST</acronym> + operators are strict and so cannot succeed for null values.) Null values + are therefore not discussed further here. + </para> + </note> + + <para> + There are five user-defined methods that an index operator class for + <acronym>SP-GiST</acronym> must provide, and two are optional. All five + mandatory methods follow the convention of accepting two <type>internal</type> + arguments, the first of which is a pointer to a C struct containing input + values for the support method, while the second argument is a pointer to a + C struct where output values must be placed. Four of the mandatory methods just + return <type>void</type>, since all their results appear in the output struct; but + <function>leaf_consistent</function> returns a <type>boolean</type> result. + The methods must not modify any fields of their input structs. In all + cases, the output struct is initialized to zeroes before calling the + user-defined method. The optional sixth method <function>compress</function> + accepts a <type>datum</type> to be indexed as the only argument and returns a value suitable + for physical storage in a leaf tuple. The optional seventh method + <function>options</function> accepts an <type>internal</type> pointer to a C struct, where + opclass-specific parameters should be placed, and returns <type>void</type>. + </para> + + <para> + The five mandatory user-defined methods are: + </para> + + <variablelist> + <varlistentry> + <term><function>config</function></term> + <listitem> + <para> + Returns static information about the index implementation, including + the data type OIDs of the prefix and node label data types. + </para> + <para> + The <acronym>SQL</acronym> declaration of the function must look like this: +<programlisting> +CREATE FUNCTION my_config(internal, internal) RETURNS void ... +</programlisting> + The first argument is a pointer to a <structname>spgConfigIn</structname> + C struct, containing input data for the function. + The second argument is a pointer to a <structname>spgConfigOut</structname> + C struct, which the function must fill with result data. +<programlisting> +typedef struct spgConfigIn +{ + Oid attType; /* Data type to be indexed */ +} spgConfigIn; + +typedef struct spgConfigOut +{ + Oid prefixType; /* Data type of inner-tuple prefixes */ + Oid labelType; /* Data type of inner-tuple node labels */ + Oid leafType; /* Data type of leaf-tuple values */ + bool canReturnData; /* Opclass can reconstruct original data */ + bool longValuesOK; /* Opclass can cope with values > 1 page */ +} spgConfigOut; +</programlisting> + + <structfield>attType</structfield> is passed in order to support polymorphic + index operator classes; for ordinary fixed-data-type operator classes, it + will always have the same value and so can be ignored. + </para> + + <para> + For operator classes that do not use prefixes, + <structfield>prefixType</structfield> can be set to <literal>VOIDOID</literal>. + Likewise, for operator classes that do not use node labels, + <structfield>labelType</structfield> can be set to <literal>VOIDOID</literal>. + <structfield>canReturnData</structfield> should be set true if the operator class + is capable of reconstructing the originally-supplied index value. + <structfield>longValuesOK</structfield> should be set true only when the + <structfield>attType</structfield> is of variable length and the operator + class is capable of segmenting long values by repeated suffixing + (see <xref linkend="spgist-limits"/>). + </para> + + <para> + <structfield>leafType</structfield> should match the index storage type + defined by the operator class's <structfield>opckeytype</structfield> + catalog entry. + (Note that <structfield>opckeytype</structfield> can be zero, + implying the storage type is the same as the operator class's input + type, which is the most common situation.) + For reasons of backward compatibility, the <function>config</function> + method can set <structfield>leafType</structfield> to some other value, + and that value will be used; but this is deprecated since the index + contents are then incorrectly identified in the catalogs. + Also, it's permissible to + leave <structfield>leafType</structfield> uninitialized (zero); + that is interpreted as meaning the index storage type derived from + <structfield>opckeytype</structfield>. + </para> + + <para> + When <structfield>attType</structfield> + and <structfield>leafType</structfield> are different, the optional + method <function>compress</function> must be provided. + Method <function>compress</function> is responsible + for transformation of datums to be indexed from <structfield>attType</structfield> + to <structfield>leafType</structfield>. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><function>choose</function></term> + <listitem> + <para> + Chooses a method for inserting a new value into an inner tuple. + </para> + + <para> + The <acronym>SQL</acronym> declaration of the function must look like this: +<programlisting> +CREATE FUNCTION my_choose(internal, internal) RETURNS void ... +</programlisting> + The first argument is a pointer to a <structname>spgChooseIn</structname> + C struct, containing input data for the function. + The second argument is a pointer to a <structname>spgChooseOut</structname> + C struct, which the function must fill with result data. +<programlisting> +typedef struct spgChooseIn +{ + Datum datum; /* original datum to be indexed */ + Datum leafDatum; /* current datum to be stored at leaf */ + int level; /* current level (counting from zero) */ + + /* Data from current inner tuple */ + bool allTheSame; /* tuple is marked all-the-same? */ + bool hasPrefix; /* tuple has a prefix? */ + Datum prefixDatum; /* if so, the prefix value */ + int nNodes; /* number of nodes in the inner tuple */ + Datum *nodeLabels; /* node label values (NULL if none) */ +} spgChooseIn; + +typedef enum spgChooseResultType +{ + spgMatchNode = 1, /* descend into existing node */ + spgAddNode, /* add a node to the inner tuple */ + spgSplitTuple /* split inner tuple (change its prefix) */ +} spgChooseResultType; + +typedef struct spgChooseOut +{ + spgChooseResultType resultType; /* action code, see above */ + union + { + struct /* results for spgMatchNode */ + { + int nodeN; /* descend to this node (index from 0) */ + int levelAdd; /* increment level by this much */ + Datum restDatum; /* new leaf datum */ + } matchNode; + struct /* results for spgAddNode */ + { + Datum nodeLabel; /* new node's label */ + int nodeN; /* where to insert it (index from 0) */ + } addNode; + struct /* results for spgSplitTuple */ + { + /* Info to form new upper-level inner tuple with one child tuple */ + bool prefixHasPrefix; /* tuple should have a prefix? */ + Datum prefixPrefixDatum; /* if so, its value */ + int prefixNNodes; /* number of nodes */ + Datum *prefixNodeLabels; /* their labels (or NULL for + * no labels) */ + int childNodeN; /* which node gets child tuple */ + + /* Info to form new lower-level inner tuple with all old nodes */ + bool postfixHasPrefix; /* tuple should have a prefix? */ + Datum postfixPrefixDatum; /* if so, its value */ + } splitTuple; + } result; +} spgChooseOut; +</programlisting> + + <structfield>datum</structfield> is the original datum of + <structname>spgConfigIn</structname>.<structfield>attType</structfield> + type that was to be inserted into the index. + <structfield>leafDatum</structfield> is a value of + <structname>spgConfigOut</structname>.<structfield>leafType</structfield> + type, which is initially a result of method + <function>compress</function> applied to <structfield>datum</structfield> + when method <function>compress</function> is provided, or the same value as + <structfield>datum</structfield> otherwise. + <structfield>leafDatum</structfield> can change at lower levels of the tree + if the <function>choose</function> or <function>picksplit</function> + methods change it. When the insertion search reaches a leaf page, + the current value of <structfield>leafDatum</structfield> is what will be stored + in the newly created leaf tuple. + <structfield>level</structfield> is the current inner tuple's level, starting at + zero for the root level. + <structfield>allTheSame</structfield> is true if the current inner tuple is + marked as containing multiple equivalent nodes + (see <xref linkend="spgist-all-the-same"/>). + <structfield>hasPrefix</structfield> is true if the current inner tuple contains + a prefix; if so, + <structfield>prefixDatum</structfield> is its value. + <structfield>nNodes</structfield> is the number of child nodes contained in the + inner tuple, and + <structfield>nodeLabels</structfield> is an array of their label values, or + NULL if there are no labels. + </para> + + <para> + The <function>choose</function> function can determine either that + the new value matches one of the existing child nodes, or that a new + child node must be added, or that the new value is inconsistent with + the tuple prefix and so the inner tuple must be split to create a + less restrictive prefix. + </para> + + <para> + If the new value matches one of the existing child nodes, + set <structfield>resultType</structfield> to <literal>spgMatchNode</literal>. + Set <structfield>nodeN</structfield> to the index (from zero) of that node in + the node array. + Set <structfield>levelAdd</structfield> to the increment in + <structfield>level</structfield> caused by descending through that node, + or leave it as zero if the operator class does not use levels. + Set <structfield>restDatum</structfield> to equal <structfield>leafDatum</structfield> + if the operator class does not modify datums from one level to the + next, or otherwise set it to the modified value to be used as + <structfield>leafDatum</structfield> at the next level. + </para> + + <para> + If a new child node must be added, + set <structfield>resultType</structfield> to <literal>spgAddNode</literal>. + Set <structfield>nodeLabel</structfield> to the label to be used for the new + node, and set <structfield>nodeN</structfield> to the index (from zero) at which + to insert the node in the node array. + After the node has been added, the <function>choose</function> + function will be called again with the modified inner tuple; + that call should result in an <literal>spgMatchNode</literal> result. + </para> + + <para> + If the new value is inconsistent with the tuple prefix, + set <structfield>resultType</structfield> to <literal>spgSplitTuple</literal>. + This action moves all the existing nodes into a new lower-level + inner tuple, and replaces the existing inner tuple with a tuple + having a single downlink pointing to the new lower-level inner tuple. + Set <structfield>prefixHasPrefix</structfield> to indicate whether the new + upper tuple should have a prefix, and if so set + <structfield>prefixPrefixDatum</structfield> to the prefix value. This new + prefix value must be sufficiently less restrictive than the original + to accept the new value to be indexed. + Set <structfield>prefixNNodes</structfield> to the number of nodes needed in the + new tuple, and set <structfield>prefixNodeLabels</structfield> to a palloc'd array + holding their labels, or to NULL if node labels are not required. + Note that the total size of the new upper tuple must be no more + than the total size of the tuple it is replacing; this constrains + the lengths of the new prefix and new labels. + Set <structfield>childNodeN</structfield> to the index (from zero) of the node + that will downlink to the new lower-level inner tuple. + Set <structfield>postfixHasPrefix</structfield> to indicate whether the new + lower-level inner tuple should have a prefix, and if so set + <structfield>postfixPrefixDatum</structfield> to the prefix value. The + combination of these two prefixes and the downlink node's label + (if any) must have the same meaning as the original prefix, because + there is no opportunity to alter the node labels that are moved to + the new lower-level tuple, nor to change any child index entries. + After the node has been split, the <function>choose</function> + function will be called again with the replacement inner tuple. + That call may return an <literal>spgAddNode</literal> result, if no suitable + node was created by the <literal>spgSplitTuple</literal> action. Eventually + <function>choose</function> must return <literal>spgMatchNode</literal> to + allow the insertion to descend to the next level. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><function>picksplit</function></term> + <listitem> + <para> + Decides how to create a new inner tuple over a set of leaf tuples. + </para> + + <para> + The <acronym>SQL</acronym> declaration of the function must look like this: +<programlisting> +CREATE FUNCTION my_picksplit(internal, internal) RETURNS void ... +</programlisting> + The first argument is a pointer to a <structname>spgPickSplitIn</structname> + C struct, containing input data for the function. + The second argument is a pointer to a <structname>spgPickSplitOut</structname> + C struct, which the function must fill with result data. +<programlisting> +typedef struct spgPickSplitIn +{ + int nTuples; /* number of leaf tuples */ + Datum *datums; /* their datums (array of length nTuples) */ + int level; /* current level (counting from zero) */ +} spgPickSplitIn; + +typedef struct spgPickSplitOut +{ + bool hasPrefix; /* new inner tuple should have a prefix? */ + Datum prefixDatum; /* if so, its value */ + + int nNodes; /* number of nodes for new inner tuple */ + Datum *nodeLabels; /* their labels (or NULL for no labels) */ + + int *mapTuplesToNodes; /* node index for each leaf tuple */ + Datum *leafTupleDatums; /* datum to store in each new leaf tuple */ +} spgPickSplitOut; +</programlisting> + + <structfield>nTuples</structfield> is the number of leaf tuples provided. + <structfield>datums</structfield> is an array of their datum values of + <structname>spgConfigOut</structname>.<structfield>leafType</structfield> + type. + <structfield>level</structfield> is the current level that all the leaf tuples + share, which will become the level of the new inner tuple. + </para> + + <para> + Set <structfield>hasPrefix</structfield> to indicate whether the new inner + tuple should have a prefix, and if so set + <structfield>prefixDatum</structfield> to the prefix value. + Set <structfield>nNodes</structfield> to indicate the number of nodes that + the new inner tuple will contain, and + set <structfield>nodeLabels</structfield> to an array of their label values, + or to NULL if node labels are not required. + Set <structfield>mapTuplesToNodes</structfield> to an array that gives the index + (from zero) of the node that each leaf tuple should be assigned to. + Set <structfield>leafTupleDatums</structfield> to an array of the values to + be stored in the new leaf tuples (these will be the same as the + input <structfield>datums</structfield> if the operator class does not modify + datums from one level to the next). + Note that the <function>picksplit</function> function is + responsible for palloc'ing the + <structfield>nodeLabels</structfield>, <structfield>mapTuplesToNodes</structfield> and + <structfield>leafTupleDatums</structfield> arrays. + </para> + + <para> + If more than one leaf tuple is supplied, it is expected that the + <function>picksplit</function> function will classify them into more than + one node; otherwise it is not possible to split the leaf tuples + across multiple pages, which is the ultimate purpose of this + operation. Therefore, if the <function>picksplit</function> function + ends up placing all the leaf tuples in the same node, the core + SP-GiST code will override that decision and generate an inner + tuple in which the leaf tuples are assigned at random to several + identically-labeled nodes. Such a tuple is marked + <literal>allTheSame</literal> to signify that this has happened. The + <function>choose</function> and <function>inner_consistent</function> functions + must take suitable care with such inner tuples. + See <xref linkend="spgist-all-the-same"/> for more information. + </para> + + <para> + <function>picksplit</function> can be applied to a single leaf tuple only + in the case that the <function>config</function> function set + <structfield>longValuesOK</structfield> to true and a larger-than-a-page input + value has been supplied. In this case the point of the operation is + to strip off a prefix and produce a new, shorter leaf datum value. + The call will be repeated until a leaf datum short enough to fit on + a page has been produced. See <xref linkend="spgist-limits"/> for + more information. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><function>inner_consistent</function></term> + <listitem> + <para> + Returns set of nodes (branches) to follow during tree search. + </para> + + <para> + The <acronym>SQL</acronym> declaration of the function must look like this: +<programlisting> +CREATE FUNCTION my_inner_consistent(internal, internal) RETURNS void ... +</programlisting> + The first argument is a pointer to a <structname>spgInnerConsistentIn</structname> + C struct, containing input data for the function. + The second argument is a pointer to a <structname>spgInnerConsistentOut</structname> + C struct, which the function must fill with result data. + +<programlisting> +typedef struct spgInnerConsistentIn +{ + ScanKey scankeys; /* array of operators and comparison values */ + ScanKey orderbys; /* array of ordering operators and comparison + * values */ + int nkeys; /* length of scankeys array */ + int norderbys; /* length of orderbys array */ + + Datum reconstructedValue; /* value reconstructed at parent */ + void *traversalValue; /* opclass-specific traverse value */ + MemoryContext traversalMemoryContext; /* put new traverse values here */ + int level; /* current level (counting from zero) */ + bool returnData; /* original data must be returned? */ + + /* Data from current inner tuple */ + bool allTheSame; /* tuple is marked all-the-same? */ + bool hasPrefix; /* tuple has a prefix? */ + Datum prefixDatum; /* if so, the prefix value */ + int nNodes; /* number of nodes in the inner tuple */ + Datum *nodeLabels; /* node label values (NULL if none) */ +} spgInnerConsistentIn; + +typedef struct spgInnerConsistentOut +{ + int nNodes; /* number of child nodes to be visited */ + int *nodeNumbers; /* their indexes in the node array */ + int *levelAdds; /* increment level by this much for each */ + Datum *reconstructedValues; /* associated reconstructed values */ + void **traversalValues; /* opclass-specific traverse values */ + double **distances; /* associated distances */ +} spgInnerConsistentOut; +</programlisting> + + The array <structfield>scankeys</structfield>, of length <structfield>nkeys</structfield>, + describes the index search condition(s). These conditions are + combined with AND — only index entries that satisfy all of + them are interesting. (Note that <structfield>nkeys</structfield> = 0 implies + that all index entries satisfy the query.) Usually the consistent + function only cares about the <structfield>sk_strategy</structfield> and + <structfield>sk_argument</structfield> fields of each array entry, which + respectively give the indexable operator and comparison value. + In particular it is not necessary to check <structfield>sk_flags</structfield> to + see if the comparison value is NULL, because the SP-GiST core code + will filter out such conditions. + The array <structfield>orderbys</structfield>, of length <structfield>norderbys</structfield>, + describes ordering operators (if any) in the same manner. + <structfield>reconstructedValue</structfield> is the value reconstructed for the + parent tuple; it is <literal>(Datum) 0</literal> at the root level or if the + <function>inner_consistent</function> function did not provide a value at the + parent level. + <structfield>traversalValue</structfield> is a pointer to any traverse data + passed down from the previous call of <function>inner_consistent</function> + on the parent index tuple, or NULL at the root level. + <structfield>traversalMemoryContext</structfield> is the memory context in which + to store output traverse values (see below). + <structfield>level</structfield> is the current inner tuple's level, starting at + zero for the root level. + <structfield>returnData</structfield> is <literal>true</literal> if reconstructed data is + required for this query; this will only be so if the + <function>config</function> function asserted <structfield>canReturnData</structfield>. + <structfield>allTheSame</structfield> is true if the current inner tuple is + marked <quote>all-the-same</quote>; in this case all the nodes have the + same label (if any) and so either all or none of them match the query + (see <xref linkend="spgist-all-the-same"/>). + <structfield>hasPrefix</structfield> is true if the current inner tuple contains + a prefix; if so, + <structfield>prefixDatum</structfield> is its value. + <structfield>nNodes</structfield> is the number of child nodes contained in the + inner tuple, and + <structfield>nodeLabels</structfield> is an array of their label values, or + NULL if the nodes do not have labels. + </para> + + <para> + <structfield>nNodes</structfield> must be set to the number of child nodes that + need to be visited by the search, and + <structfield>nodeNumbers</structfield> must be set to an array of their indexes. + If the operator class keeps track of levels, set + <structfield>levelAdds</structfield> to an array of the level increments + required when descending to each node to be visited. (Often these + increments will be the same for all the nodes, but that's not + necessarily so, so an array is used.) + If value reconstruction is needed, set + <structfield>reconstructedValues</structfield> to an array of the values + reconstructed for each child node to be visited; otherwise, leave + <structfield>reconstructedValues</structfield> as NULL. + The reconstructed values are assumed to be of type + <structname>spgConfigOut</structname>.<structfield>leafType</structfield>. + (However, since the core system will do nothing with them except + possibly copy them, it is sufficient for them to have the + same <literal>typlen</literal> and <literal>typbyval</literal> + properties as <structfield>leafType</structfield>.) + If ordered search is performed, set <structfield>distances</structfield> + to an array of distance values according to <structfield>orderbys</structfield> + array (nodes with lowest distances will be processed first). Leave it + NULL otherwise. + If it is desired to pass down additional out-of-band information + (<quote>traverse values</quote>) to lower levels of the tree search, + set <structfield>traversalValues</structfield> to an array of the appropriate + traverse values, one for each child node to be visited; otherwise, + leave <structfield>traversalValues</structfield> as NULL. + Note that the <function>inner_consistent</function> function is + responsible for palloc'ing the + <structfield>nodeNumbers</structfield>, <structfield>levelAdds</structfield>, + <structfield>distances</structfield>, + <structfield>reconstructedValues</structfield>, and + <structfield>traversalValues</structfield> arrays in the current memory context. + However, any output traverse values pointed to by + the <structfield>traversalValues</structfield> array should be allocated + in <structfield>traversalMemoryContext</structfield>. + Each traverse value must be a single palloc'd chunk. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><function>leaf_consistent</function></term> + <listitem> + <para> + Returns true if a leaf tuple satisfies a query. + </para> + + <para> + The <acronym>SQL</acronym> declaration of the function must look like this: +<programlisting> +CREATE FUNCTION my_leaf_consistent(internal, internal) RETURNS bool ... +</programlisting> + The first argument is a pointer to a <structname>spgLeafConsistentIn</structname> + C struct, containing input data for the function. + The second argument is a pointer to a <structname>spgLeafConsistentOut</structname> + C struct, which the function must fill with result data. +<programlisting> +typedef struct spgLeafConsistentIn +{ + ScanKey scankeys; /* array of operators and comparison values */ + ScanKey orderbys; /* array of ordering operators and comparison + * values */ + int nkeys; /* length of scankeys array */ + int norderbys; /* length of orderbys array */ + + Datum reconstructedValue; /* value reconstructed at parent */ + void *traversalValue; /* opclass-specific traverse value */ + int level; /* current level (counting from zero) */ + bool returnData; /* original data must be returned? */ + + Datum leafDatum; /* datum in leaf tuple */ +} spgLeafConsistentIn; + +typedef struct spgLeafConsistentOut +{ + Datum leafValue; /* reconstructed original data, if any */ + bool recheck; /* set true if operator must be rechecked */ + bool recheckDistances; /* set true if distances must be rechecked */ + double *distances; /* associated distances */ +} spgLeafConsistentOut; +</programlisting> + + The array <structfield>scankeys</structfield>, of length <structfield>nkeys</structfield>, + describes the index search condition(s). These conditions are + combined with AND — only index entries that satisfy all of + them satisfy the query. (Note that <structfield>nkeys</structfield> = 0 implies + that all index entries satisfy the query.) Usually the consistent + function only cares about the <structfield>sk_strategy</structfield> and + <structfield>sk_argument</structfield> fields of each array entry, which + respectively give the indexable operator and comparison value. + In particular it is not necessary to check <structfield>sk_flags</structfield> to + see if the comparison value is NULL, because the SP-GiST core code + will filter out such conditions. + The array <structfield>orderbys</structfield>, of length <structfield>norderbys</structfield>, + describes the ordering operators in the same manner. + <structfield>reconstructedValue</structfield> is the value reconstructed for the + parent tuple; it is <literal>(Datum) 0</literal> at the root level or if the + <function>inner_consistent</function> function did not provide a value at the + parent level. + <structfield>traversalValue</structfield> is a pointer to any traverse data + passed down from the previous call of <function>inner_consistent</function> + on the parent index tuple, or NULL at the root level. + <structfield>level</structfield> is the current leaf tuple's level, starting at + zero for the root level. + <structfield>returnData</structfield> is <literal>true</literal> if reconstructed data is + required for this query; this will only be so if the + <function>config</function> function asserted <structfield>canReturnData</structfield>. + <structfield>leafDatum</structfield> is the key value of + <structname>spgConfigOut</structname>.<structfield>leafType</structfield> + stored in the current leaf tuple. + </para> + + <para> + The function must return <literal>true</literal> if the leaf tuple matches the + query, or <literal>false</literal> if not. In the <literal>true</literal> case, + if <structfield>returnData</structfield> is <literal>true</literal> then + <structfield>leafValue</structfield> must be set to the value (of type + <structname>spgConfigIn</structname>.<structfield>attType</structfield>) + originally supplied to be indexed for this leaf tuple. Also, + <structfield>recheck</structfield> may be set to <literal>true</literal> if the match + is uncertain and so the operator(s) must be re-applied to the actual + heap tuple to verify the match. + If ordered search is performed, set <structfield>distances</structfield> + to an array of distance values according to <structfield>orderbys</structfield> + array. Leave it NULL otherwise. If at least one of returned distances + is not exact, set <structfield>recheckDistances</structfield> to true. + In this case, the executor will calculate the exact distances after + fetching the tuple from the heap, and will reorder the tuples if needed. + </para> + </listitem> + </varlistentry> + </variablelist> + + <para> + The optional user-defined methods are: + </para> + + <variablelist> + <varlistentry> + <term><function>Datum compress(Datum in)</function></term> + <listitem> + <para> + Converts a data item into a format suitable for physical storage in + a leaf tuple of the index. It accepts a value of type + <structname>spgConfigIn</structname>.<structfield>attType</structfield> + and returns a value of type + <structname>spgConfigOut</structname>.<structfield>leafType</structfield>. + The output value must not contain an out-of-line TOAST pointer. + </para> + + <para> + Note: the <function>compress</function> method is only applied to + values to be stored. The consistent methods receive query + <structfield>scankeys</structfield> unchanged, without transformation + using <function>compress</function>. + </para> + </listitem> + </varlistentry> + + <varlistentry> + <term><function>options</function></term> + <listitem> + <para> + Defines a set of user-visible parameters that control operator class + behavior. + </para> + + <para> + The <acronym>SQL</acronym> declaration of the function must look like this: + +<programlisting> +CREATE OR REPLACE FUNCTION my_options(internal) +RETURNS void +AS 'MODULE_PATHNAME' +LANGUAGE C STRICT; +</programlisting> + </para> + + <para> + The function is passed a pointer to a <structname>local_relopts</structname> + struct, which needs to be filled with a set of operator class + specific options. The options can be accessed from other support + functions using the <literal>PG_HAS_OPCLASS_OPTIONS()</literal> and + <literal>PG_GET_OPCLASS_OPTIONS()</literal> macros. + </para> + + <para> + Since the representation of the key in <acronym>SP-GiST</acronym> is + flexible, it may depend on user-specified parameters. + </para> + </listitem> + </varlistentry> + </variablelist> + + <para> + All the SP-GiST support methods are normally called in a short-lived + memory context; that is, <varname>CurrentMemoryContext</varname> will be reset + after processing of each tuple. It is therefore not very important to + worry about pfree'ing everything you palloc. (The <function>config</function> + method is an exception: it should try to avoid leaking memory. But + usually the <function>config</function> method need do nothing but assign + constants into the passed parameter struct.) + </para> + + <para> + If the indexed column is of a collatable data type, the index collation + will be passed to all the support methods, using the standard + <function>PG_GET_COLLATION()</function> mechanism. + </para> + +</sect1> + +<sect1 id="spgist-implementation"> + <title>Implementation</title> + + <para> + This section covers implementation details and other tricks that are + useful for implementers of <acronym>SP-GiST</acronym> operator classes to + know. + </para> + + <sect2 id="spgist-limits"> + <title>SP-GiST Limits</title> + + <para> + Individual leaf tuples and inner tuples must fit on a single index page + (8kB by default). Therefore, when indexing values of variable-length + data types, long values can only be supported by methods such as radix + trees, in which each level of the tree includes a prefix that is short + enough to fit on a page, and the final leaf level includes a suffix also + short enough to fit on a page. The operator class should set + <structfield>longValuesOK</structfield> to true only if it is prepared to arrange for + this to happen. Otherwise, the <acronym>SP-GiST</acronym> core will + reject any request to index a value that is too large to fit + on an index page. + </para> + + <para> + Likewise, it is the operator class's responsibility that inner tuples + do not grow too large to fit on an index page; this limits the number + of child nodes that can be used in one inner tuple, as well as the + maximum size of a prefix value. + </para> + + <para> + Another limitation is that when an inner tuple's node points to a set + of leaf tuples, those tuples must all be in the same index page. + (This is a design decision to reduce seeking and save space in the + links that chain such tuples together.) If the set of leaf tuples + grows too large for a page, a split is performed and an intermediate + inner tuple is inserted. For this to fix the problem, the new inner + tuple <emphasis>must</emphasis> divide the set of leaf values into more than one + node group. If the operator class's <function>picksplit</function> function + fails to do that, the <acronym>SP-GiST</acronym> core resorts to + extraordinary measures described in <xref linkend="spgist-all-the-same"/>. + </para> + + <para> + When <structfield>longValuesOK</structfield> is true, it is expected + that successive levels of the <acronym>SP-GiST</acronym> tree will + absorb more and more information into the prefixes and node labels of + the inner tuples, making the required leaf datum smaller and smaller, + so that eventually it will fit on a page. + To prevent bugs in operator classes from causing infinite insertion + loops, the <acronym>SP-GiST</acronym> core will raise an error if the + leaf datum does not become any smaller within ten cycles + of <function>choose</function> method calls. + </para> + </sect2> + + <sect2 id="spgist-null-labels"> + <title>SP-GiST Without Node Labels</title> + + <para> + Some tree algorithms use a fixed set of nodes for each inner tuple; + for example, in a quad-tree there are always exactly four nodes + corresponding to the four quadrants around the inner tuple's centroid + point. In such a case the code typically works with the nodes by + number, and there is no need for explicit node labels. To suppress + node labels (and thereby save some space), the <function>picksplit</function> + function can return NULL for the <structfield>nodeLabels</structfield> array, + and likewise the <function>choose</function> function can return NULL for + the <structfield>prefixNodeLabels</structfield> array during + a <literal>spgSplitTuple</literal> action. + This will in turn result in <structfield>nodeLabels</structfield> being NULL during + subsequent calls to <function>choose</function> and <function>inner_consistent</function>. + In principle, node labels could be used for some inner tuples and omitted + for others in the same index. + </para> + + <para> + When working with an inner tuple having unlabeled nodes, it is an error + for <function>choose</function> to return <literal>spgAddNode</literal>, since the set + of nodes is supposed to be fixed in such cases. + </para> + </sect2> + + <sect2 id="spgist-all-the-same"> + <title><quote>All-the-Same</quote> Inner Tuples</title> + + <para> + The <acronym>SP-GiST</acronym> core can override the results of the + operator class's <function>picksplit</function> function when + <function>picksplit</function> fails to divide the supplied leaf values into + at least two node categories. When this happens, the new inner tuple + is created with multiple nodes that each have the same label (if any) + that <function>picksplit</function> gave to the one node it did use, and the + leaf values are divided at random among these equivalent nodes. + The <literal>allTheSame</literal> flag is set on the inner tuple to warn the + <function>choose</function> and <function>inner_consistent</function> functions that the + tuple does not have the node set that they might otherwise expect. + </para> + + <para> + When dealing with an <literal>allTheSame</literal> tuple, a <function>choose</function> + result of <literal>spgMatchNode</literal> is interpreted to mean that the new + value can be assigned to any of the equivalent nodes; the core code will + ignore the supplied <structfield>nodeN</structfield> value and descend into one + of the nodes at random (so as to keep the tree balanced). It is an + error for <function>choose</function> to return <literal>spgAddNode</literal>, since + that would make the nodes not all equivalent; the + <literal>spgSplitTuple</literal> action must be used if the value to be inserted + doesn't match the existing nodes. + </para> + + <para> + When dealing with an <literal>allTheSame</literal> tuple, the + <function>inner_consistent</function> function should return either all or none + of the nodes as targets for continuing the index search, since they are + all equivalent. This may or may not require any special-case code, + depending on how much the <function>inner_consistent</function> function normally + assumes about the meaning of the nodes. + </para> + </sect2> + +</sect1> + +<sect1 id="spgist-examples"> + <title>Examples</title> + + <para> + The <productname>PostgreSQL</productname> source distribution includes + several examples of index operator classes for <acronym>SP-GiST</acronym>, + as described in <xref linkend="spgist-builtin-opclasses-table"/>. Look + into <filename>src/backend/access/spgist/</filename> + and <filename>src/backend/utils/adt/</filename> to see the code. + </para> + +</sect1> + +</chapter> |