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diff --git a/doc/src/sgml/html/gin-extensibility.html b/doc/src/sgml/html/gin-extensibility.html new file mode 100644 index 0000000..21fc2a3 --- /dev/null +++ b/doc/src/sgml/html/gin-extensibility.html @@ -0,0 +1,237 @@ +<?xml version="1.0" encoding="UTF-8" standalone="no"?> +<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>67.3. Extensibility</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="gin-builtin-opclasses.html" title="67.2. Built-in Operator Classes" /><link rel="next" href="gin-implementation.html" title="67.4. Implementation" /></head><body id="docContent" class="container-fluid col-10"><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">67.3. Extensibility</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="gin-builtin-opclasses.html" title="67.2. Built-in Operator Classes">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="gin.html" title="Chapter 67. GIN Indexes">Up</a></td><th width="60%" align="center">Chapter 67. GIN Indexes</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 14.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="gin-implementation.html" title="67.4. Implementation">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="GIN-EXTENSIBILITY"><div class="titlepage"><div><div><h2 class="title" style="clear: both">67.3. Extensibility</h2></div></div></div><p> + The <acronym class="acronym">GIN</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 class="acronym">GIN</acronym> layer itself + takes care of concurrency, logging and searching the tree structure. + </p><p> + All it takes to get a <acronym class="acronym">GIN</acronym> access method working is to + implement a few user-defined methods, which define the behavior of + keys in the tree and the relationships between keys, indexed items, + and indexable queries. In short, <acronym class="acronym">GIN</acronym> combines + extensibility with generality, code reuse, and a clean interface. + </p><p> + There are two methods that an operator class for + <acronym class="acronym">GIN</acronym> must provide: + + </p><div class="variablelist"><dl class="variablelist"><dt><span class="term"><code class="function">Datum *extractValue(Datum itemValue, int32 *nkeys, + bool **nullFlags)</code></span></dt><dd><p> + Returns a palloc'd array of keys given an item to be indexed. The + number of returned keys must be stored into <code class="literal">*nkeys</code>. + If any of the keys can be null, also palloc an array of + <code class="literal">*nkeys</code> <code class="type">bool</code> fields, store its address at + <code class="literal">*nullFlags</code>, and set these null flags as needed. + <code class="literal">*nullFlags</code> can be left <code class="symbol">NULL</code> (its initial value) + if all keys are non-null. + The return value can be <code class="symbol">NULL</code> if the item contains no keys. + </p></dd><dt><span class="term"><code class="function">Datum *extractQuery(Datum query, int32 *nkeys, + StrategyNumber n, bool **pmatch, Pointer **extra_data, + bool **nullFlags, int32 *searchMode)</code></span></dt><dd><p> + Returns a palloc'd array of keys given a value to be queried; that is, + <code class="literal">query</code> is the value on the right-hand side of an + indexable operator whose left-hand side is the indexed column. + <code class="literal">n</code> is the strategy number of the operator within the + operator class (see <a class="xref" href="xindex.html#XINDEX-STRATEGIES" title="38.16.2. Index Method Strategies">Section 38.16.2</a>). + Often, <code class="function">extractQuery</code> will need + to consult <code class="literal">n</code> to determine the data type of + <code class="literal">query</code> and the method it should use to extract key values. + The number of returned keys must be stored into <code class="literal">*nkeys</code>. + If any of the keys can be null, also palloc an array of + <code class="literal">*nkeys</code> <code class="type">bool</code> fields, store its address at + <code class="literal">*nullFlags</code>, and set these null flags as needed. + <code class="literal">*nullFlags</code> can be left <code class="symbol">NULL</code> (its initial value) + if all keys are non-null. + The return value can be <code class="symbol">NULL</code> if the <code class="literal">query</code> contains no keys. + </p><p> + <code class="literal">searchMode</code> is an output argument that allows + <code class="function">extractQuery</code> to specify details about how the search + will be done. + If <code class="literal">*searchMode</code> is set to + <code class="literal">GIN_SEARCH_MODE_DEFAULT</code> (which is the value it is + initialized to before call), only items that match at least one of + the returned keys are considered candidate matches. + If <code class="literal">*searchMode</code> is set to + <code class="literal">GIN_SEARCH_MODE_INCLUDE_EMPTY</code>, then in addition to items + containing at least one matching key, items that contain no keys at + all are considered candidate matches. (This mode is useful for + implementing is-subset-of operators, for example.) + If <code class="literal">*searchMode</code> is set to <code class="literal">GIN_SEARCH_MODE_ALL</code>, + then all non-null items in the index are considered candidate + matches, whether they match any of the returned keys or not. (This + mode is much slower than the other two choices, since it requires + scanning essentially the entire index, but it may be necessary to + implement corner cases correctly. An operator that needs this mode + in most cases is probably not a good candidate for a GIN operator + class.) + The symbols to use for setting this mode are defined in + <code class="filename">access/gin.h</code>. + </p><p> + <code class="literal">pmatch</code> is an output argument for use when partial match + is supported. To use it, <code class="function">extractQuery</code> must allocate + an array of <code class="literal">*nkeys</code> <code class="type">bool</code>s and store its address at + <code class="literal">*pmatch</code>. Each element of the array should be set to true + if the corresponding key requires partial match, false if not. + If <code class="literal">*pmatch</code> is set to <code class="symbol">NULL</code> then GIN assumes partial match + is not required. The variable is initialized to <code class="symbol">NULL</code> before call, + so this argument can simply be ignored by operator classes that do + not support partial match. + </p><p> + <code class="literal">extra_data</code> is an output argument that allows + <code class="function">extractQuery</code> to pass additional data to the + <code class="function">consistent</code> and <code class="function">comparePartial</code> methods. + To use it, <code class="function">extractQuery</code> must allocate + an array of <code class="literal">*nkeys</code> pointers and store its address at + <code class="literal">*extra_data</code>, then store whatever it wants to into the + individual pointers. The variable is initialized to <code class="symbol">NULL</code> before + call, so this argument can simply be ignored by operator classes that + do not require extra data. If <code class="literal">*extra_data</code> is set, the + whole array is passed to the <code class="function">consistent</code> method, and + the appropriate element to the <code class="function">comparePartial</code> method. + </p></dd></dl></div><p> + + An operator class must also provide a function to check if an indexed item + matches the query. It comes in two flavors, a Boolean <code class="function">consistent</code> + function, and a ternary <code class="function">triConsistent</code> function. + <code class="function">triConsistent</code> covers the functionality of both, so providing + <code class="function">triConsistent</code> alone is sufficient. However, if the Boolean + variant is significantly cheaper to calculate, it can be advantageous to + provide both. If only the Boolean variant is provided, some optimizations + that depend on refuting index items before fetching all the keys are + disabled. + + </p><div class="variablelist"><dl class="variablelist"><dt><span class="term"><code class="function">bool consistent(bool check[], StrategyNumber n, Datum query, + int32 nkeys, Pointer extra_data[], bool *recheck, + Datum queryKeys[], bool nullFlags[])</code></span></dt><dd><p> + Returns true if an indexed item satisfies the query operator with + strategy number <code class="literal">n</code> (or might satisfy it, if the recheck + indication is returned). This function does not have direct access + to the indexed item's value, since <acronym class="acronym">GIN</acronym> does not + store items explicitly. Rather, what is available is knowledge + about which key values extracted from the query appear in a given + indexed item. The <code class="literal">check</code> array has length + <code class="literal">nkeys</code>, which is the same as the number of keys previously + returned by <code class="function">extractQuery</code> for this <code class="literal">query</code> datum. + Each element of the + <code class="literal">check</code> array is true if the indexed item contains the + corresponding query key, i.e., if (check[i] == true) the i-th key of the + <code class="function">extractQuery</code> result array is present in the indexed item. + The original <code class="literal">query</code> datum is + passed in case the <code class="function">consistent</code> method needs to consult it, + and so are the <code class="literal">queryKeys[]</code> and <code class="literal">nullFlags[]</code> + arrays previously returned by <code class="function">extractQuery</code>. + <code class="literal">extra_data</code> is the extra-data array returned by + <code class="function">extractQuery</code>, or <code class="symbol">NULL</code> if none. + </p><p> + When <code class="function">extractQuery</code> returns a null key in + <code class="literal">queryKeys[]</code>, the corresponding <code class="literal">check[]</code> element + is true if the indexed item contains a null key; that is, the + semantics of <code class="literal">check[]</code> are like <code class="literal">IS NOT DISTINCT + FROM</code>. The <code class="function">consistent</code> function can examine the + corresponding <code class="literal">nullFlags[]</code> element if it needs to tell + the difference between a regular value match and a null match. + </p><p> + On success, <code class="literal">*recheck</code> should be set to true if the heap + tuple needs to be rechecked against the query operator, or false if + the index test is exact. That is, a false return value guarantees + that the heap tuple does not match the query; a true return value with + <code class="literal">*recheck</code> set to false guarantees that the heap tuple does + match the query; and a true return value with + <code class="literal">*recheck</code> set to true means that the heap tuple might match + the query, so it needs to be fetched and rechecked by evaluating the + query operator directly against the originally indexed item. + </p></dd><dt><span class="term"><code class="function">GinTernaryValue triConsistent(GinTernaryValue check[], StrategyNumber n, Datum query, + int32 nkeys, Pointer extra_data[], + Datum queryKeys[], bool nullFlags[])</code></span></dt><dd><p> + <code class="function">triConsistent</code> is similar to <code class="function">consistent</code>, + but instead of Booleans in the <code class="literal">check</code> vector, there are + three possible values for each + key: <code class="literal">GIN_TRUE</code>, <code class="literal">GIN_FALSE</code> and + <code class="literal">GIN_MAYBE</code>. <code class="literal">GIN_FALSE</code> and <code class="literal">GIN_TRUE</code> + have the same meaning as regular Boolean values, while + <code class="literal">GIN_MAYBE</code> means that the presence of that key is not known. + When <code class="literal">GIN_MAYBE</code> values are present, the function should only + return <code class="literal">GIN_TRUE</code> if the item certainly matches whether or + not the index item contains the corresponding query keys. Likewise, the + function must return <code class="literal">GIN_FALSE</code> only if the item certainly + does not match, whether or not it contains the <code class="literal">GIN_MAYBE</code> + keys. If the result depends on the <code class="literal">GIN_MAYBE</code> entries, i.e., + the match cannot be confirmed or refuted based on the known query keys, + the function must return <code class="literal">GIN_MAYBE</code>. + </p><p> + When there are no <code class="literal">GIN_MAYBE</code> values in the <code class="literal">check</code> + vector, a <code class="literal">GIN_MAYBE</code> return value is the equivalent of + setting the <code class="literal">recheck</code> flag in the + Boolean <code class="function">consistent</code> function. + </p></dd></dl></div><p> + </p><p> + In addition, GIN must have a way to sort the key values stored in the index. + The operator class can define the sort ordering by specifying a comparison + method: + + </p><div class="variablelist"><dl class="variablelist"><dt><span class="term"><code class="function">int compare(Datum a, Datum b)</code></span></dt><dd><p> + Compares two keys (not indexed items!) and returns an integer less than + zero, zero, or greater than zero, indicating whether the first key is + less than, equal to, or greater than the second. Null keys are never + passed to this function. + </p></dd></dl></div><p> + + Alternatively, if the operator class does not provide a <code class="function">compare</code> + method, GIN will look up the default btree operator class for the index + key data type, and use its comparison function. It is recommended to + specify the comparison function in a GIN operator class that is meant for + just one data type, as looking up the btree operator class costs a few + cycles. However, polymorphic GIN operator classes (such + as <code class="literal">array_ops</code>) typically cannot specify a single comparison + function. + </p><p> + An operator class for <acronym class="acronym">GIN</acronym> can optionally supply the + following methods: + + </p><div class="variablelist"><dl class="variablelist"><dt><span class="term"><code class="function">int comparePartial(Datum partial_key, Datum key, StrategyNumber n, + Pointer extra_data)</code></span></dt><dd><p> + Compare a partial-match query key to an index key. Returns an integer + whose sign indicates the result: less than zero means the index key + does not match the query, but the index scan should continue; zero + means that the index key does match the query; greater than zero + indicates that the index scan should stop because no more matches + are possible. The strategy number <code class="literal">n</code> of the operator + that generated the partial match query is provided, in case its + semantics are needed to determine when to end the scan. Also, + <code class="literal">extra_data</code> is the corresponding element of the extra-data + array made by <code class="function">extractQuery</code>, or <code class="symbol">NULL</code> if none. + Null keys are never passed to this function. + </p></dd><dt><span class="term"><code class="function">void options(local_relopts *relopts)</code></span></dt><dd><p> + Defines a set of user-visible parameters that control operator class + behavior. + </p><p> + The <code class="function">options</code> function is passed a pointer to a + <code class="structname">local_relopts</code> 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 + <code class="literal">PG_HAS_OPCLASS_OPTIONS()</code> and + <code class="literal">PG_GET_OPCLASS_OPTIONS()</code> macros. + </p><p> + Since both key extraction of indexed values and representation of the + key in <acronym class="acronym">GIN</acronym> are flexible, they may depend on + user-specified parameters. + </p></dd></dl></div><p> + </p><p> + To support <span class="quote">“<span class="quote">partial match</span>”</span> queries, an operator class must + provide the <code class="function">comparePartial</code> method, and its + <code class="function">extractQuery</code> method must set the <code class="literal">pmatch</code> + parameter when a partial-match query is encountered. See + <a class="xref" href="gin-implementation.html#GIN-PARTIAL-MATCH" title="67.4.2. Partial Match Algorithm">Section 67.4.2</a> for details. + </p><p> + The actual data types of the various <code class="literal">Datum</code> values mentioned + above vary depending on the operator class. The item values passed to + <code class="function">extractValue</code> are always of the operator class's input type, and + all key values must be of the class's <code class="literal">STORAGE</code> type. The type of + the <code class="literal">query</code> argument passed to <code class="function">extractQuery</code>, + <code class="function">consistent</code> and <code class="function">triConsistent</code> is whatever is the + right-hand input type of the class member operator identified by the + strategy number. This need not be the same as the indexed type, so long as + key values of the correct type can be extracted from it. However, it is + recommended that the SQL declarations of these three support functions use + the opclass's indexed data type for the <code class="literal">query</code> argument, even + though the actual type might be something else depending on the operator. + </p></div><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navfooter"><hr></hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="gin-builtin-opclasses.html" title="67.2. Built-in Operator Classes">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="gin.html" title="Chapter 67. GIN Indexes">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="gin-implementation.html" title="67.4. Implementation">Next</a></td></tr><tr><td width="40%" align="left" valign="top">67.2. 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