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<!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>66.3. 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 V1.79.1" /><link rel="prev" href="gin-builtin-opclasses.html" title="66.2. Built-in Operator Classes" /><link rel="next" href="gin-implementation.html" title="66.4. Implementation" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">66.3. Extensibility</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="gin-builtin-opclasses.html" title="66.2. Built-in Operator Classes">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="gin.html" title="Chapter 66. GIN Indexes">Up</a></td><th width="60%" align="center">Chapter 66. GIN Indexes</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 12.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="gin-implementation.html" title="66.4. 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">66.3. 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="37.16.2. Index Method Strategies">Section 37.16.2</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>
  Optionally, an operator class for <acronym class="acronym">GIN</acronym> can supply the
  following method:

  </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></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="66.4.2. Partial Match Algorithm">Section 66.4.2</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 class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="gin-builtin-opclasses.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="gin.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="gin-implementation.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">66.2. Built-in Operator Classes </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> 66.4. Implementation</td></tr></table></div></body></html>