summaryrefslogtreecommitdiffstats
path: root/doc/src/sgml/gist.sgml
diff options
context:
space:
mode:
Diffstat (limited to 'doc/src/sgml/gist.sgml')
-rw-r--r--doc/src/sgml/gist.sgml1065
1 files changed, 1065 insertions, 0 deletions
diff --git a/doc/src/sgml/gist.sgml b/doc/src/sgml/gist.sgml
new file mode 100644
index 0000000..46982ba
--- /dev/null
+++ b/doc/src/sgml/gist.sgml
@@ -0,0 +1,1065 @@
+<!-- doc/src/sgml/gist.sgml -->
+
+<chapter id="gist">
+<title>GiST Indexes</title>
+
+ <indexterm>
+ <primary>index</primary>
+ <secondary>GiST</secondary>
+ </indexterm>
+
+<sect1 id="gist-intro">
+ <title>Introduction</title>
+
+ <para>
+ <acronym>GiST</acronym> stands for Generalized Search Tree. It is a
+ balanced, tree-structured access method, that acts as a base template in
+ which to implement arbitrary indexing schemes. B-trees, R-trees and many
+ other indexing schemes can be implemented in <acronym>GiST</acronym>.
+ </para>
+
+ <para>
+ One advantage of <acronym>GiST</acronym> is that it allows 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 the University of California
+ at Berkeley's GiST Indexing Project
+ <ulink url="http://gist.cs.berkeley.edu/">web site</ulink> and
+ Marcel Kornacker's thesis,
+ <ulink url="http://www.sai.msu.su/~megera/postgres/gist/papers/concurrency/access-methods-for-next-generation.pdf.gz">
+ Access Methods for Next-Generation Database Systems</ulink>.
+ The <acronym>GiST</acronym>
+ implementation in <productname>PostgreSQL</productname> is primarily
+ maintained by Teodor Sigaev and Oleg Bartunov, and there is more
+ information on their
+ <ulink url="http://www.sai.msu.su/~megera/postgres/gist/">web site</ulink>.
+ </para>
+
+</sect1>
+
+<sect1 id="gist-builtin-opclasses">
+ <title>Built-in Operator Classes</title>
+
+ <para>
+ The core <productname>PostgreSQL</productname> distribution
+ includes the <acronym>GiST</acronym> operator classes shown in
+ <xref linkend="gist-builtin-opclasses-table"/>.
+ (Some of the optional modules described in <xref linkend="contrib"/>
+ provide additional <acronym>GiST</acronym> operator classes.)
+ </para>
+
+ <table id="gist-builtin-opclasses-table">
+ <title>Built-in <acronym>GiST</acronym> Operator Classes</title>
+ <tgroup cols="4">
+ <thead>
+ <row>
+ <entry>Name</entry>
+ <entry>Indexed Data Type</entry>
+ <entry>Indexable Operators</entry>
+ <entry>Ordering Operators</entry>
+ </row>
+ </thead>
+ <tbody>
+ <row>
+ <entry><literal>box_ops</literal></entry>
+ <entry><type>box</type></entry>
+ <entry>
+ <literal>&amp;&amp;</literal>
+ <literal>&amp;&gt;</literal>
+ <literal>&amp;&lt;</literal>
+ <literal>&amp;&lt;|</literal>
+ <literal>&gt;&gt;</literal>
+ <literal>&lt;&lt;</literal>
+ <literal>&lt;&lt;|</literal>
+ <literal>&lt;@</literal>
+ <literal>@&gt;</literal>
+ <literal>@</literal>
+ <literal>|&amp;&gt;</literal>
+ <literal>|&gt;&gt;</literal>
+ <literal>~</literal>
+ <literal>~=</literal>
+ </entry>
+ <entry>
+ </entry>
+ </row>
+ <row>
+ <entry><literal>circle_ops</literal></entry>
+ <entry><type>circle</type></entry>
+ <entry>
+ <literal>&amp;&amp;</literal>
+ <literal>&amp;&gt;</literal>
+ <literal>&amp;&lt;</literal>
+ <literal>&amp;&lt;|</literal>
+ <literal>&gt;&gt;</literal>
+ <literal>&lt;&lt;</literal>
+ <literal>&lt;&lt;|</literal>
+ <literal>&lt;@</literal>
+ <literal>@&gt;</literal>
+ <literal>@</literal>
+ <literal>|&amp;&gt;</literal>
+ <literal>|&gt;&gt;</literal>
+ <literal>~</literal>
+ <literal>~=</literal>
+ </entry>
+ <entry>
+ <literal>&lt;-&gt;</literal>
+ </entry>
+ </row>
+ <row>
+ <entry><literal>inet_ops</literal></entry>
+ <entry><type>inet</type>, <type>cidr</type></entry>
+ <entry>
+ <literal>&amp;&amp;</literal>
+ <literal>&gt;&gt;</literal>
+ <literal>&gt;&gt;=</literal>
+ <literal>&gt;</literal>
+ <literal>&gt;=</literal>
+ <literal>&lt;&gt;</literal>
+ <literal>&lt;&lt;</literal>
+ <literal>&lt;&lt;=</literal>
+ <literal>&lt;</literal>
+ <literal>&lt;=</literal>
+ <literal>=</literal>
+ </entry>
+ <entry>
+ </entry>
+ </row>
+ <row>
+ <entry><literal>point_ops</literal></entry>
+ <entry><type>point</type></entry>
+ <entry>
+ <literal>&gt;&gt;</literal>
+ <literal>&gt;^</literal>
+ <literal>&lt;&lt;</literal>
+ <literal>&lt;@</literal>
+ <literal>&lt;@</literal>
+ <literal>&lt;@</literal>
+ <literal>&lt;^</literal>
+ <literal>~=</literal>
+ </entry>
+ <entry>
+ <literal>&lt;-&gt;</literal>
+ </entry>
+ </row>
+ <row>
+ <entry><literal>poly_ops</literal></entry>
+ <entry><type>polygon</type></entry>
+ <entry>
+ <literal>&amp;&amp;</literal>
+ <literal>&amp;&gt;</literal>
+ <literal>&amp;&lt;</literal>
+ <literal>&amp;&lt;|</literal>
+ <literal>&gt;&gt;</literal>
+ <literal>&lt;&lt;</literal>
+ <literal>&lt;&lt;|</literal>
+ <literal>&lt;@</literal>
+ <literal>@&gt;</literal>
+ <literal>@</literal>
+ <literal>|&amp;&gt;</literal>
+ <literal>|&gt;&gt;</literal>
+ <literal>~</literal>
+ <literal>~=</literal>
+ </entry>
+ <entry>
+ <literal>&lt;-&gt;</literal>
+ </entry>
+ </row>
+ <row>
+ <entry><literal>range_ops</literal></entry>
+ <entry>any range type</entry>
+ <entry>
+ <literal>&amp;&amp;</literal>
+ <literal>&amp;&gt;</literal>
+ <literal>&amp;&lt;</literal>
+ <literal>&gt;&gt;</literal>
+ <literal>&lt;&lt;</literal>
+ <literal>&lt;@</literal>
+ <literal>-|-</literal>
+ <literal>=</literal>
+ <literal>@&gt;</literal>
+ <literal>@&gt;</literal>
+ </entry>
+ <entry>
+ </entry>
+ </row>
+ <row>
+ <entry><literal>tsquery_ops</literal></entry>
+ <entry><type>tsquery</type></entry>
+ <entry>
+ <literal>&lt;@</literal>
+ <literal>@&gt;</literal>
+ </entry>
+ <entry>
+ </entry>
+ </row>
+ <row>
+ <entry><literal>tsvector_ops</literal></entry>
+ <entry><type>tsvector</type></entry>
+ <entry>
+ <literal>@@</literal>
+ </entry>
+ <entry>
+ </entry>
+ </row>
+ </tbody>
+ </tgroup>
+ </table>
+
+ <para>
+ For historical reasons, the <literal>inet_ops</literal> operator class is
+ not the default class for types <type>inet</type> and <type>cidr</type>.
+ To use it, mention the class name in <command>CREATE INDEX</command>,
+ for example
+<programlisting>
+CREATE INDEX ON my_table USING GIST (my_inet_column inet_ops);
+</programlisting>
+ </para>
+
+</sect1>
+
+<sect1 id="gist-extensibility">
+ <title>Extensibility</title>
+
+ <para>
+ Traditionally, implementing a new index access method meant a lot of
+ difficult work. It was necessary to understand the inner workings of the
+ database, such as the lock manager and Write-Ahead Log. The
+ <acronym>GiST</acronym> interface has a high level of abstraction,
+ requiring the access method implementer only to implement the semantics of
+ the data type being accessed. The <acronym>GiST</acronym> layer itself
+ takes care of concurrency, logging and searching the tree structure.
+ </para>
+
+ <para>
+ This extensibility should not be confused with the extensibility of the
+ other standard search trees in terms of the data they can handle. For
+ example, <productname>PostgreSQL</productname> supports extensible B-trees
+ and hash indexes. That means that you can use
+ <productname>PostgreSQL</productname> to build a B-tree or hash over any
+ data type you want. But B-trees only support range predicates
+ (<literal>&lt;</literal>, <literal>=</literal>, <literal>&gt;</literal>),
+ and hash indexes only support equality queries.
+ </para>
+
+ <para>
+ So if you index, say, an image collection with a
+ <productname>PostgreSQL</productname> B-tree, you can only issue queries
+ such as <quote>is imagex equal to imagey</quote>, <quote>is imagex less
+ than imagey</quote> and <quote>is imagex greater than imagey</quote>.
+ Depending on how you define <quote>equals</quote>, <quote>less than</quote>
+ and <quote>greater than</quote> in this context, this could be useful.
+ However, by using a <acronym>GiST</acronym> based index, you could create
+ ways to ask domain-specific questions, perhaps <quote>find all images of
+ horses</quote> or <quote>find all over-exposed images</quote>.
+ </para>
+
+ <para>
+ All it takes to get a <acronym>GiST</acronym> access method up and running
+ is to implement several user-defined methods, which define the behavior of
+ keys in the tree. Of course these methods have to be pretty fancy to
+ support fancy queries, but for all the standard queries (B-trees,
+ R-trees, etc.) they're relatively straightforward. In short,
+ <acronym>GiST</acronym> combines extensibility along with generality, code
+ reuse, and a clean interface.
+ </para>
+
+ <para>
+ There are five methods that an index operator class for
+ <acronym>GiST</acronym> must provide, and four that are optional.
+ Correctness of the index is ensured
+ by proper implementation of the <function>same</function>, <function>consistent</function>
+ and <function>union</function> methods, while efficiency (size and speed) of the
+ index will depend on the <function>penalty</function> and <function>picksplit</function>
+ methods.
+ Two optional methods are <function>compress</function> and
+ <function>decompress</function>, which allow an index to have internal tree data of
+ a different type than the data it indexes. The leaves are to be of the
+ indexed data type, while the other tree nodes can be of any C struct (but
+ you still have to follow <productname>PostgreSQL</productname> data type rules here,
+ see about <literal>varlena</literal> for variable sized data). If the tree's
+ internal data type exists at the SQL level, the <literal>STORAGE</literal> option
+ of the <command>CREATE OPERATOR CLASS</command> command can be used.
+ The optional eighth method is <function>distance</function>, which is needed
+ if the operator class wishes to support ordered scans (nearest-neighbor
+ searches). The optional ninth method <function>fetch</function> is needed if the
+ operator class wishes to support index-only scans, except when the
+ <function>compress</function> method is omitted.
+ </para>
+
+ <variablelist>
+ <varlistentry>
+ <term><function>consistent</function></term>
+ <listitem>
+ <para>
+ Given an index entry <literal>p</literal> and a query value <literal>q</literal>,
+ this function determines whether the index entry is
+ <quote>consistent</quote> with the query; that is, could the predicate
+ <quote><replaceable>indexed_column</replaceable>
+ <replaceable>indexable_operator</replaceable> <literal>q</literal></quote> be true for
+ any row represented by the index entry? For a leaf index entry this is
+ equivalent to testing the indexable condition, while for an internal
+ tree node this determines whether it is necessary to scan the subtree
+ of the index represented by the tree node. When the result is
+ <literal>true</literal>, a <literal>recheck</literal> flag must also be returned.
+ This indicates whether the predicate is certainly true or only possibly
+ true. If <literal>recheck</literal> = <literal>false</literal> then the index has
+ tested the predicate condition exactly, whereas if <literal>recheck</literal>
+ = <literal>true</literal> the row is only a candidate match. In that case the
+ system will automatically evaluate the
+ <replaceable>indexable_operator</replaceable> against the actual row value to see
+ if it is really a match. This convention allows
+ <acronym>GiST</acronym> to support both lossless and lossy index
+ structures.
+ </para>
+
+ <para>
+ The <acronym>SQL</acronym> declaration of the function must look like this:
+
+<programlisting>
+CREATE OR REPLACE FUNCTION my_consistent(internal, data_type, smallint, oid, internal)
+RETURNS bool
+AS 'MODULE_PATHNAME'
+LANGUAGE C STRICT;
+</programlisting>
+
+ And the matching code in the C module could then follow this skeleton:
+
+<programlisting>
+PG_FUNCTION_INFO_V1(my_consistent);
+
+Datum
+my_consistent(PG_FUNCTION_ARGS)
+{
+ GISTENTRY *entry = (GISTENTRY *) PG_GETARG_POINTER(0);
+ data_type *query = PG_GETARG_DATA_TYPE_P(1);
+ StrategyNumber strategy = (StrategyNumber) PG_GETARG_UINT16(2);
+ /* Oid subtype = PG_GETARG_OID(3); */
+ bool *recheck = (bool *) PG_GETARG_POINTER(4);
+ data_type *key = DatumGetDataType(entry-&gt;key);
+ bool retval;
+
+ /*
+ * determine return value as a function of strategy, key and query.
+ *
+ * Use GIST_LEAF(entry) to know where you're called in the index tree,
+ * which comes handy when supporting the = operator for example (you could
+ * check for non empty union() in non-leaf nodes and equality in leaf
+ * nodes).
+ */
+
+ *recheck = true; /* or false if check is exact */
+
+ PG_RETURN_BOOL(retval);
+}
+</programlisting>
+
+ Here, <varname>key</varname> is an element in the index and <varname>query</varname>
+ the value being looked up in the index. The <literal>StrategyNumber</literal>
+ parameter indicates which operator of your operator class is being
+ applied &mdash; it matches one of the operator numbers in the
+ <command>CREATE OPERATOR CLASS</command> command.
+ </para>
+
+ <para>
+ Depending on which operators you have included in the class, the data
+ type of <varname>query</varname> could vary with the operator, since it will
+ be whatever type is on the righthand side of the operator, which might
+ be different from the indexed data type appearing on the lefthand side.
+ (The above code skeleton assumes that only one type is possible; if
+ not, fetching the <varname>query</varname> argument value would have to depend
+ on the operator.) It is recommended that the SQL declaration of
+ the <function>consistent</function> function use the opclass's indexed data
+ type for the <varname>query</varname> argument, even though the actual type
+ might be something else depending on the operator.
+ </para>
+
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><function>union</function></term>
+ <listitem>
+ <para>
+ This method consolidates information in the tree. Given a set of
+ entries, this function generates a new index entry that represents
+ all the given entries.
+ </para>
+
+ <para>
+ The <acronym>SQL</acronym> declaration of the function must look like this:
+
+<programlisting>
+CREATE OR REPLACE FUNCTION my_union(internal, internal)
+RETURNS storage_type
+AS 'MODULE_PATHNAME'
+LANGUAGE C STRICT;
+</programlisting>
+
+ And the matching code in the C module could then follow this skeleton:
+
+<programlisting>
+PG_FUNCTION_INFO_V1(my_union);
+
+Datum
+my_union(PG_FUNCTION_ARGS)
+{
+ GistEntryVector *entryvec = (GistEntryVector *) PG_GETARG_POINTER(0);
+ GISTENTRY *ent = entryvec-&gt;vector;
+ data_type *out,
+ *tmp,
+ *old;
+ int numranges,
+ i = 0;
+
+ numranges = entryvec-&gt;n;
+ tmp = DatumGetDataType(ent[0].key);
+ out = tmp;
+
+ if (numranges == 1)
+ {
+ out = data_type_deep_copy(tmp);
+
+ PG_RETURN_DATA_TYPE_P(out);
+ }
+
+ for (i = 1; i &lt; numranges; i++)
+ {
+ old = out;
+ tmp = DatumGetDataType(ent[i].key);
+ out = my_union_implementation(out, tmp);
+ }
+
+ PG_RETURN_DATA_TYPE_P(out);
+}
+</programlisting>
+ </para>
+
+ <para>
+ As you can see, in this skeleton we're dealing with a data type
+ where <literal>union(X, Y, Z) = union(union(X, Y), Z)</literal>. It's easy
+ enough to support data types where this is not the case, by
+ implementing the proper union algorithm in this
+ <acronym>GiST</acronym> support method.
+ </para>
+
+ <para>
+ The result of the <function>union</function> function must be a value of the
+ index's storage type, whatever that is (it might or might not be
+ different from the indexed column's type). The <function>union</function>
+ function should return a pointer to newly <function>palloc()</function>ed
+ memory. You can't just return the input value as-is, even if there is
+ no type change.
+ </para>
+
+ <para>
+ As shown above, the <function>union</function> function's
+ first <type>internal</type> argument is actually
+ a <structname>GistEntryVector</structname> pointer. The second argument is a
+ pointer to an integer variable, which can be ignored. (It used to be
+ required that the <function>union</function> function store the size of its
+ result value into that variable, but this is no longer necessary.)
+ </para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><function>compress</function></term>
+ <listitem>
+ <para>
+ Converts a data item into a format suitable for physical storage in
+ an index page.
+ If the <function>compress</function> method is omitted, data items are stored
+ in the index without modification.
+ </para>
+
+ <para>
+ The <acronym>SQL</acronym> declaration of the function must look like this:
+
+<programlisting>
+CREATE OR REPLACE FUNCTION my_compress(internal)
+RETURNS internal
+AS 'MODULE_PATHNAME'
+LANGUAGE C STRICT;
+</programlisting>
+
+ And the matching code in the C module could then follow this skeleton:
+
+<programlisting>
+PG_FUNCTION_INFO_V1(my_compress);
+
+Datum
+my_compress(PG_FUNCTION_ARGS)
+{
+ GISTENTRY *entry = (GISTENTRY *) PG_GETARG_POINTER(0);
+ GISTENTRY *retval;
+
+ if (entry-&gt;leafkey)
+ {
+ /* replace entry-&gt;key with a compressed version */
+ compressed_data_type *compressed_data = palloc(sizeof(compressed_data_type));
+
+ /* fill *compressed_data from entry-&gt;key ... */
+
+ retval = palloc(sizeof(GISTENTRY));
+ gistentryinit(*retval, PointerGetDatum(compressed_data),
+ entry-&gt;rel, entry-&gt;page, entry-&gt;offset, FALSE);
+ }
+ else
+ {
+ /* typically we needn't do anything with non-leaf entries */
+ retval = entry;
+ }
+
+ PG_RETURN_POINTER(retval);
+}
+</programlisting>
+ </para>
+
+ <para>
+ You have to adapt <replaceable>compressed_data_type</replaceable> to the specific
+ type you're converting to in order to compress your leaf nodes, of
+ course.
+ </para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><function>decompress</function></term>
+ <listitem>
+ <para>
+ Converts the stored representation of a data item into a format that
+ can be manipulated by the other GiST methods in the operator class.
+ If the <function>decompress</function> method is omitted, it is assumed that
+ the other GiST methods can work directly on the stored data format.
+ (<function>decompress</function> is not necessarily the reverse of
+ the <function>compress</function> method; in particular,
+ if <function>compress</function> is lossy then it's impossible
+ for <function>decompress</function> to exactly reconstruct the original
+ data. <function>decompress</function> is not necessarily equivalent
+ to <function>fetch</function>, either, since the other GiST methods might not
+ require full reconstruction of the data.)
+ </para>
+
+ <para>
+ The <acronym>SQL</acronym> declaration of the function must look like this:
+
+<programlisting>
+CREATE OR REPLACE FUNCTION my_decompress(internal)
+RETURNS internal
+AS 'MODULE_PATHNAME'
+LANGUAGE C STRICT;
+</programlisting>
+
+ And the matching code in the C module could then follow this skeleton:
+
+<programlisting>
+PG_FUNCTION_INFO_V1(my_decompress);
+
+Datum
+my_decompress(PG_FUNCTION_ARGS)
+{
+ PG_RETURN_POINTER(PG_GETARG_POINTER(0));
+}
+</programlisting>
+
+ The above skeleton is suitable for the case where no decompression
+ is needed. (But, of course, omitting the method altogether is even
+ easier, and is recommended in such cases.)
+ </para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><function>penalty</function></term>
+ <listitem>
+ <para>
+ Returns a value indicating the <quote>cost</quote> of inserting the new
+ entry into a particular branch of the tree. Items will be inserted
+ down the path of least <function>penalty</function> in the tree.
+ Values returned by <function>penalty</function> should be non-negative.
+ If a negative value is returned, it will be treated as zero.
+ </para>
+
+ <para>
+ The <acronym>SQL</acronym> declaration of the function must look like this:
+
+<programlisting>
+CREATE OR REPLACE FUNCTION my_penalty(internal, internal, internal)
+RETURNS internal
+AS 'MODULE_PATHNAME'
+LANGUAGE C STRICT; -- in some cases penalty functions need not be strict
+</programlisting>
+
+ And the matching code in the C module could then follow this skeleton:
+
+<programlisting>
+PG_FUNCTION_INFO_V1(my_penalty);
+
+Datum
+my_penalty(PG_FUNCTION_ARGS)
+{
+ GISTENTRY *origentry = (GISTENTRY *) PG_GETARG_POINTER(0);
+ GISTENTRY *newentry = (GISTENTRY *) PG_GETARG_POINTER(1);
+ float *penalty = (float *) PG_GETARG_POINTER(2);
+ data_type *orig = DatumGetDataType(origentry-&gt;key);
+ data_type *new = DatumGetDataType(newentry-&gt;key);
+
+ *penalty = my_penalty_implementation(orig, new);
+ PG_RETURN_POINTER(penalty);
+}
+</programlisting>
+
+ For historical reasons, the <function>penalty</function> function doesn't
+ just return a <type>float</type> result; instead it has to store the value
+ at the location indicated by the third argument. The return
+ value per se is ignored, though it's conventional to pass back the
+ address of that argument.
+ </para>
+
+ <para>
+ The <function>penalty</function> function is crucial to good performance of
+ the index. It'll get used at insertion time to determine which branch
+ to follow when choosing where to add the new entry in the tree. At
+ query time, the more balanced the index, the quicker the lookup.
+ </para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><function>picksplit</function></term>
+ <listitem>
+ <para>
+ When an index page split is necessary, this function decides which
+ entries on the page are to stay on the old page, and which are to move
+ to the new page.
+ </para>
+
+ <para>
+ The <acronym>SQL</acronym> declaration of the function must look like this:
+
+<programlisting>
+CREATE OR REPLACE FUNCTION my_picksplit(internal, internal)
+RETURNS internal
+AS 'MODULE_PATHNAME'
+LANGUAGE C STRICT;
+</programlisting>
+
+ And the matching code in the C module could then follow this skeleton:
+
+<programlisting>
+PG_FUNCTION_INFO_V1(my_picksplit);
+
+Datum
+my_picksplit(PG_FUNCTION_ARGS)
+{
+ GistEntryVector *entryvec = (GistEntryVector *) PG_GETARG_POINTER(0);
+ GIST_SPLITVEC *v = (GIST_SPLITVEC *) PG_GETARG_POINTER(1);
+ OffsetNumber maxoff = entryvec-&gt;n - 1;
+ GISTENTRY *ent = entryvec-&gt;vector;
+ int i,
+ nbytes;
+ OffsetNumber *left,
+ *right;
+ data_type *tmp_union;
+ data_type *unionL;
+ data_type *unionR;
+ GISTENTRY **raw_entryvec;
+
+ maxoff = entryvec-&gt;n - 1;
+ nbytes = (maxoff + 1) * sizeof(OffsetNumber);
+
+ v-&gt;spl_left = (OffsetNumber *) palloc(nbytes);
+ left = v-&gt;spl_left;
+ v-&gt;spl_nleft = 0;
+
+ v-&gt;spl_right = (OffsetNumber *) palloc(nbytes);
+ right = v-&gt;spl_right;
+ v-&gt;spl_nright = 0;
+
+ unionL = NULL;
+ unionR = NULL;
+
+ /* Initialize the raw entry vector. */
+ raw_entryvec = (GISTENTRY **) malloc(entryvec-&gt;n * sizeof(void *));
+ for (i = FirstOffsetNumber; i &lt;= maxoff; i = OffsetNumberNext(i))
+ raw_entryvec[i] = &amp;(entryvec-&gt;vector[i]);
+
+ for (i = FirstOffsetNumber; i &lt;= maxoff; i = OffsetNumberNext(i))
+ {
+ int real_index = raw_entryvec[i] - entryvec-&gt;vector;
+
+ tmp_union = DatumGetDataType(entryvec-&gt;vector[real_index].key);
+ Assert(tmp_union != NULL);
+
+ /*
+ * Choose where to put the index entries and update unionL and unionR
+ * accordingly. Append the entries to either v-&gt;spl_left or
+ * v-&gt;spl_right, and care about the counters.
+ */
+
+ if (my_choice_is_left(unionL, curl, unionR, curr))
+ {
+ if (unionL == NULL)
+ unionL = tmp_union;
+ else
+ unionL = my_union_implementation(unionL, tmp_union);
+
+ *left = real_index;
+ ++left;
+ ++(v-&gt;spl_nleft);
+ }
+ else
+ {
+ /*
+ * Same on the right
+ */
+ }
+ }
+
+ v-&gt;spl_ldatum = DataTypeGetDatum(unionL);
+ v-&gt;spl_rdatum = DataTypeGetDatum(unionR);
+ PG_RETURN_POINTER(v);
+}
+</programlisting>
+
+ Notice that the <function>picksplit</function> function's result is delivered
+ by modifying the passed-in <structname>v</structname> structure. The return
+ value per se is ignored, though it's conventional to pass back the
+ address of <structname>v</structname>.
+ </para>
+
+ <para>
+ Like <function>penalty</function>, the <function>picksplit</function> function
+ is crucial to good performance of the index. Designing suitable
+ <function>penalty</function> and <function>picksplit</function> implementations
+ is where the challenge of implementing well-performing
+ <acronym>GiST</acronym> indexes lies.
+ </para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><function>same</function></term>
+ <listitem>
+ <para>
+ Returns true if two index entries are identical, false otherwise.
+ (An <quote>index entry</quote> is a value of the index's storage type,
+ not necessarily the original indexed column's type.)
+ </para>
+
+ <para>
+ The <acronym>SQL</acronym> declaration of the function must look like this:
+
+<programlisting>
+CREATE OR REPLACE FUNCTION my_same(storage_type, storage_type, internal)
+RETURNS internal
+AS 'MODULE_PATHNAME'
+LANGUAGE C STRICT;
+</programlisting>
+
+ And the matching code in the C module could then follow this skeleton:
+
+<programlisting>
+PG_FUNCTION_INFO_V1(my_same);
+
+Datum
+my_same(PG_FUNCTION_ARGS)
+{
+ prefix_range *v1 = PG_GETARG_PREFIX_RANGE_P(0);
+ prefix_range *v2 = PG_GETARG_PREFIX_RANGE_P(1);
+ bool *result = (bool *) PG_GETARG_POINTER(2);
+
+ *result = my_eq(v1, v2);
+ PG_RETURN_POINTER(result);
+}
+</programlisting>
+
+ For historical reasons, the <function>same</function> function doesn't
+ just return a Boolean result; instead it has to store the flag
+ at the location indicated by the third argument. The return
+ value per se is ignored, though it's conventional to pass back the
+ address of that argument.
+ </para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><function>distance</function></term>
+ <listitem>
+ <para>
+ Given an index entry <literal>p</literal> and a query value <literal>q</literal>,
+ this function determines the index entry's
+ <quote>distance</quote> from the query value. This function must be
+ supplied if the operator class contains any ordering operators.
+ A query using the ordering operator will be implemented by returning
+ index entries with the smallest <quote>distance</quote> values first,
+ so the results must be consistent with the operator's semantics.
+ For a leaf index entry the result just represents the distance to
+ the index entry; for an internal tree node, the result must be the
+ smallest distance that any child entry could have.
+ </para>
+
+ <para>
+ The <acronym>SQL</acronym> declaration of the function must look like this:
+
+<programlisting>
+CREATE OR REPLACE FUNCTION my_distance(internal, data_type, smallint, oid, internal)
+RETURNS float8
+AS 'MODULE_PATHNAME'
+LANGUAGE C STRICT;
+</programlisting>
+
+ And the matching code in the C module could then follow this skeleton:
+
+<programlisting>
+PG_FUNCTION_INFO_V1(my_distance);
+
+Datum
+my_distance(PG_FUNCTION_ARGS)
+{
+ GISTENTRY *entry = (GISTENTRY *) PG_GETARG_POINTER(0);
+ data_type *query = PG_GETARG_DATA_TYPE_P(1);
+ StrategyNumber strategy = (StrategyNumber) PG_GETARG_UINT16(2);
+ /* Oid subtype = PG_GETARG_OID(3); */
+ /* bool *recheck = (bool *) PG_GETARG_POINTER(4); */
+ data_type *key = DatumGetDataType(entry-&gt;key);
+ double retval;
+
+ /*
+ * determine return value as a function of strategy, key and query.
+ */
+
+ PG_RETURN_FLOAT8(retval);
+}
+</programlisting>
+
+ The arguments to the <function>distance</function> function are identical to
+ the arguments of the <function>consistent</function> function.
+ </para>
+
+ <para>
+ Some approximation is allowed when determining the distance, so long
+ as the result is never greater than the entry's actual distance. Thus,
+ for example, distance to a bounding box is usually sufficient in
+ geometric applications. For an internal tree node, the distance
+ returned must not be greater than the distance to any of the child
+ nodes. If the returned distance is not exact, the function must set
+ <literal>*recheck</literal> to true. (This is not necessary for internal tree
+ nodes; for them, the calculation is always assumed to be inexact.) In
+ this case the executor will calculate the accurate distance after
+ fetching the tuple from the heap, and reorder the tuples if necessary.
+ </para>
+
+ <para>
+ If the distance function returns <literal>*recheck = true</literal> for any
+ leaf node, the original ordering operator's return type must
+ be <type>float8</type> or <type>float4</type>, and the distance function's
+ result values must be comparable to those of the original ordering
+ operator, since the executor will sort using both distance function
+ results and recalculated ordering-operator results. Otherwise, the
+ distance function's result values can be any finite <type>float8</type>
+ values, so long as the relative order of the result values matches the
+ order returned by the ordering operator. (Infinity and minus infinity
+ are used internally to handle cases such as nulls, so it is not
+ recommended that <function>distance</function> functions return these values.)
+ </para>
+
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><function>fetch</function></term>
+ <listitem>
+ <para>
+ Converts the compressed index representation of a data item into the
+ original data type, for index-only scans. The returned data must be an
+ exact, non-lossy copy of the originally indexed value.
+ </para>
+
+ <para>
+ The <acronym>SQL</acronym> declaration of the function must look like this:
+
+<programlisting>
+CREATE OR REPLACE FUNCTION my_fetch(internal)
+RETURNS internal
+AS 'MODULE_PATHNAME'
+LANGUAGE C STRICT;
+</programlisting>
+
+ The argument is a pointer to a <structname>GISTENTRY</structname> struct. On
+ entry, its <structfield>key</structfield> field contains a non-NULL leaf datum in
+ compressed form. The return value is another <structname>GISTENTRY</structname>
+ struct, whose <structfield>key</structfield> field contains the same datum in its
+ original, uncompressed form. If the opclass's compress function does
+ nothing for leaf entries, the <function>fetch</function> method can return the
+ argument as-is. Or, if the opclass does not have a compress function,
+ the <function>fetch</function> method can be omitted as well, since it would
+ necessarily be a no-op.
+ </para>
+
+ <para>
+ The matching code in the C module could then follow this skeleton:
+
+<programlisting>
+PG_FUNCTION_INFO_V1(my_fetch);
+
+Datum
+my_fetch(PG_FUNCTION_ARGS)
+{
+ GISTENTRY *entry = (GISTENTRY *) PG_GETARG_POINTER(0);
+ input_data_type *in = DatumGetPointer(entry->key);
+ fetched_data_type *fetched_data;
+ GISTENTRY *retval;
+
+ retval = palloc(sizeof(GISTENTRY));
+ fetched_data = palloc(sizeof(fetched_data_type));
+
+ /*
+ * Convert 'fetched_data' into the a Datum of the original datatype.
+ */
+
+ /* fill *retval from fetched_data. */
+ gistentryinit(*retval, PointerGetDatum(converted_datum),
+ entry->rel, entry->page, entry->offset, FALSE);
+
+ PG_RETURN_POINTER(retval);
+}
+</programlisting>
+ </para>
+
+ <para>
+ If the compress method is lossy for leaf entries, the operator class
+ cannot support index-only scans, and must not define
+ a <function>fetch</function> function.
+ </para>
+
+ </listitem>
+ </varlistentry>
+ </variablelist>
+
+ <para>
+ All the GiST support methods are normally called in short-lived memory
+ contexts; that is, <varname>CurrentMemoryContext</varname> will get reset after
+ each tuple is processed. It is therefore not very important to worry about
+ pfree'ing everything you palloc. However, in some cases it's useful for a
+ support method to cache data across repeated calls. To do that, allocate
+ the longer-lived data in <literal>fcinfo-&gt;flinfo-&gt;fn_mcxt</literal>, and
+ keep a pointer to it in <literal>fcinfo-&gt;flinfo-&gt;fn_extra</literal>. Such
+ data will survive for the life of the index operation (e.g., a single GiST
+ index scan, index build, or index tuple insertion). Be careful to pfree
+ the previous value when replacing a <literal>fn_extra</literal> value, or the leak
+ will accumulate for the duration of the operation.
+ </para>
+
+</sect1>
+
+<sect1 id="gist-implementation">
+ <title>Implementation</title>
+
+ <sect2 id="gist-buffering-build">
+ <title>GiST Buffering Build</title>
+ <para>
+ Building large GiST indexes by simply inserting all the tuples tends to be
+ slow, because if the index tuples are scattered across the index and the
+ index is large enough to not fit in cache, the insertions need to perform
+ a lot of random I/O. Beginning in version 9.2, PostgreSQL supports a more
+ efficient method to build GiST indexes based on buffering, which can
+ dramatically reduce the number of random I/Os needed for non-ordered data
+ sets. For well-ordered data sets the benefit is smaller or non-existent,
+ because only a small number of pages receive new tuples at a time, and
+ those pages fit in cache even if the index as whole does not.
+ </para>
+
+ <para>
+ However, buffering index build needs to call the <function>penalty</function>
+ function more often, which consumes some extra CPU resources. Also, the
+ buffers used in the buffering build need temporary disk space, up to
+ the size of the resulting index. Buffering can also influence the quality
+ of the resulting index, in both positive and negative directions. That
+ influence depends on various factors, like the distribution of the input
+ data and the operator class implementation.
+ </para>
+
+ <para>
+ By default, a GiST index build switches to the buffering method when the
+ index size reaches <xref linkend="guc-effective-cache-size"/>. It can
+ be manually turned on or off by the <literal>buffering</literal> parameter
+ to the CREATE INDEX command. The default behavior is good for most cases,
+ but turning buffering off might speed up the build somewhat if the input
+ data is ordered.
+ </para>
+
+ </sect2>
+</sect1>
+
+<sect1 id="gist-examples">
+ <title>Examples</title>
+
+ <para>
+ The <productname>PostgreSQL</productname> source distribution includes
+ several examples of index methods implemented using
+ <acronym>GiST</acronym>. The core system currently provides text search
+ support (indexing for <type>tsvector</type> and <type>tsquery</type>) as well as
+ R-Tree equivalent functionality for some of the built-in geometric data types
+ (see <filename>src/backend/access/gist/gistproc.c</filename>). The following
+ <filename>contrib</filename> modules also contain <acronym>GiST</acronym>
+ operator classes:
+
+ <variablelist>
+ <varlistentry>
+ <term><filename>btree_gist</filename></term>
+ <listitem>
+ <para>B-tree equivalent functionality for several data types</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><filename>cube</filename></term>
+ <listitem>
+ <para>Indexing for multidimensional cubes</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><filename>hstore</filename></term>
+ <listitem>
+ <para>Module for storing (key, value) pairs</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><filename>intarray</filename></term>
+ <listitem>
+ <para>RD-Tree for one-dimensional array of int4 values</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><filename>ltree</filename></term>
+ <listitem>
+ <para>Indexing for tree-like structures</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><filename>pg_trgm</filename></term>
+ <listitem>
+ <para>Text similarity using trigram matching</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term><filename>seg</filename></term>
+ <listitem>
+ <para>Indexing for <quote>float ranges</quote></para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+ </para>
+
+</sect1>
+
+</chapter>