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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>61.2. Index Access Method Functions</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="index-api.html" title="61.1. Basic API Structure for Indexes" /><link rel="next" href="index-scanning.html" title="61.3. Index Scanning" /></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">61.2. Index Access Method Functions</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="index-api.html" title="61.1. Basic API Structure for Indexes">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="indexam.html" title="Chapter 61. Index Access Method Interface Definition">Up</a></td><th width="60%" align="center">Chapter 61. Index Access Method Interface Definition</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="index-scanning.html" title="61.3. Index Scanning">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="INDEX-FUNCTIONS"><div class="titlepage"><div><div><h2 class="title" style="clear: both">61.2. Index Access Method Functions</h2></div></div></div><p>
   The index construction and maintenance functions that an index access
   method must provide in <code class="structname">IndexAmRoutine</code> are:
  </p><p>
</p><pre class="programlisting">
IndexBuildResult *
ambuild (Relation heapRelation,
         Relation indexRelation,
         IndexInfo *indexInfo);
</pre><p>
   Build a new index.  The index relation has been physically created,
   but is empty.  It must be filled in with whatever fixed data the
   access method requires, plus entries for all tuples already existing
   in the table.  Ordinarily the <code class="function">ambuild</code> function will call
   <code class="function">table_index_build_scan()</code> to scan the table for existing tuples
   and compute the keys that need to be inserted into the index.
   The function must return a palloc'd struct containing statistics about
   the new index.
  </p><p>
</p><pre class="programlisting">
void
ambuildempty (Relation indexRelation);
</pre><p>
   Build an empty index, and write it to the initialization fork (<code class="symbol">INIT_FORKNUM</code>)
   of the given relation.  This method is called only for unlogged indexes; the
   empty index written to the initialization fork will be copied over the main
   relation fork on each server restart.
  </p><p>
</p><pre class="programlisting">
bool
aminsert (Relation indexRelation,
          Datum *values,
          bool *isnull,
          ItemPointer heap_tid,
          Relation heapRelation,
          IndexUniqueCheck checkUnique,
          IndexInfo *indexInfo);
</pre><p>
   Insert a new tuple into an existing index.  The <code class="literal">values</code> and
   <code class="literal">isnull</code> arrays give the key values to be indexed, and
   <code class="literal">heap_tid</code> is the TID to be indexed.
   If the access method supports unique indexes (its
   <code class="structfield">amcanunique</code> flag is true) then
   <code class="literal">checkUnique</code> indicates the type of uniqueness check to
   perform.  This varies depending on whether the unique constraint is
   deferrable; see <a class="xref" href="index-unique-checks.html" title="61.5. Index Uniqueness Checks">Section 61.5</a> for details.
   Normally the access method only needs the <code class="literal">heapRelation</code>
   parameter when performing uniqueness checking (since then it will have to
   look into the heap to verify tuple liveness).
  </p><p>
   The function's Boolean result value is significant only when
   <code class="literal">checkUnique</code> is <code class="literal">UNIQUE_CHECK_PARTIAL</code>.
   In this case a true result means the new entry is known unique, whereas
   false means it might be non-unique (and a deferred uniqueness check must
   be scheduled).  For other cases a constant false result is recommended.
  </p><p>
   Some indexes might not index all tuples.  If the tuple is not to be
   indexed, <code class="function">aminsert</code> should just return without doing anything.
  </p><p>
   If the index AM wishes to cache data across successive index insertions
   within a SQL statement, it can allocate space
   in <code class="literal">indexInfo-&gt;ii_Context</code> and store a pointer to the
   data in <code class="literal">indexInfo-&gt;ii_AmCache</code> (which will be NULL
   initially).
  </p><p>
</p><pre class="programlisting">
IndexBulkDeleteResult *
ambulkdelete (IndexVacuumInfo *info,
              IndexBulkDeleteResult *stats,
              IndexBulkDeleteCallback callback,
              void *callback_state);
</pre><p>
   Delete tuple(s) from the index.  This is a <span class="quote">“<span class="quote">bulk delete</span>”</span> operation
   that is intended to be implemented by scanning the whole index and checking
   each entry to see if it should be deleted.
   The passed-in <code class="literal">callback</code> function must be called, in the style
   <code class="literal">callback(<em class="replaceable"><code>TID</code></em>, callback_state) returns bool</code>,
   to determine whether any particular index entry, as identified by its
   referenced TID, is to be deleted.  Must return either NULL or a palloc'd
   struct containing statistics about the effects of the deletion operation.
   It is OK to return NULL if no information needs to be passed on to
   <code class="function">amvacuumcleanup</code>.
  </p><p>
   Because of limited <code class="varname">maintenance_work_mem</code>,
   <code class="function">ambulkdelete</code> might need to be called more than once when many
   tuples are to be deleted.  The <code class="literal">stats</code> argument is the result
   of the previous call for this index (it is NULL for the first call within a
   <code class="command">VACUUM</code> operation).  This allows the AM to accumulate statistics
   across the whole operation.  Typically, <code class="function">ambulkdelete</code> will
   modify and return the same struct if the passed <code class="literal">stats</code> is not
   null.
  </p><p>
</p><pre class="programlisting">
IndexBulkDeleteResult *
amvacuumcleanup (IndexVacuumInfo *info,
                 IndexBulkDeleteResult *stats);
</pre><p>
   Clean up after a <code class="command">VACUUM</code> operation (zero or more
   <code class="function">ambulkdelete</code> calls).  This does not have to do anything
   beyond returning index statistics, but it might perform bulk cleanup
   such as reclaiming empty index pages.  <code class="literal">stats</code> is whatever the
   last <code class="function">ambulkdelete</code> call returned, or NULL if
   <code class="function">ambulkdelete</code> was not called because no tuples needed to be
   deleted.  If the result is not NULL it must be a palloc'd struct.
   The statistics it contains will be used to update <code class="structname">pg_class</code>,
   and will be reported by <code class="command">VACUUM</code> if <code class="literal">VERBOSE</code> is given.
   It is OK to return NULL if the index was not changed at all during the
   <code class="command">VACUUM</code> operation, but otherwise correct stats should
   be returned.
  </p><p>
   As of <span class="productname">PostgreSQL</span> 8.4,
   <code class="function">amvacuumcleanup</code> will also be called at completion of an
   <code class="command">ANALYZE</code> operation.  In this case <code class="literal">stats</code> is always
   NULL and any return value will be ignored.  This case can be distinguished
   by checking <code class="literal">info-&gt;analyze_only</code>.  It is recommended
   that the access method do nothing except post-insert cleanup in such a
   call, and that only in an autovacuum worker process.
  </p><p>
</p><pre class="programlisting">
bool
amcanreturn (Relation indexRelation, int attno);
</pre><p>
   Check whether the index can support <a class="link" href="indexes-index-only-scans.html" title="11.9. Index-Only Scans and Covering Indexes"><em class="firstterm">index-only scans</em></a> on
   the given column, by returning the indexed column values for an index entry
   in the form of an <code class="structname">IndexTuple</code>.  The attribute number
   is 1-based, i.e. the first column's attno is 1. Returns true if supported,
   else false.  If the access method does not support index-only scans at all,
   the <code class="structfield">amcanreturn</code> field in its <code class="structname">IndexAmRoutine</code>
   struct can be set to NULL.
  </p><p>
</p><pre class="programlisting">
void
amcostestimate (PlannerInfo *root,
                IndexPath *path,
                double loop_count,
                Cost *indexStartupCost,
                Cost *indexTotalCost,
                Selectivity *indexSelectivity,
                double *indexCorrelation,
                double *indexPages);
</pre><p>
   Estimate the costs of an index scan.  This function is described fully
   in <a class="xref" href="index-cost-estimation.html" title="61.6. Index Cost Estimation Functions">Section 61.6</a>, below.
  </p><p>
</p><pre class="programlisting">
bytea *
amoptions (ArrayType *reloptions,
           bool validate);
</pre><p>
   Parse and validate the reloptions array for an index.  This is called only
   when a non-null reloptions array exists for the index.
   <em class="parameter"><code>reloptions</code></em> is a <code class="type">text</code> array containing entries of the
   form <em class="replaceable"><code>name</code></em><code class="literal">=</code><em class="replaceable"><code>value</code></em>.
   The function should construct a <code class="type">bytea</code> value, which will be copied
   into the <code class="structfield">rd_options</code> field of the index's relcache entry.
   The data contents of the <code class="type">bytea</code> value are open for the access
   method to define; most of the standard access methods use struct
   <code class="structname">StdRdOptions</code>.
   When <em class="parameter"><code>validate</code></em> is true, the function should report a suitable
   error message if any of the options are unrecognized or have invalid
   values; when <em class="parameter"><code>validate</code></em> is false, invalid entries should be
   silently ignored.  (<em class="parameter"><code>validate</code></em> is false when loading options
   already stored in <code class="structname">pg_catalog</code>; an invalid entry could only
   be found if the access method has changed its rules for options, and in
   that case ignoring obsolete entries is appropriate.)
   It is OK to return NULL if default behavior is wanted.
  </p><p>
</p><pre class="programlisting">
bool
amproperty (Oid index_oid, int attno,
            IndexAMProperty prop, const char *propname,
            bool *res, bool *isnull);
</pre><p>
   The <code class="function">amproperty</code> method allows index access methods to override
   the default behavior of <code class="function">pg_index_column_has_property</code>
   and related functions.
   If the access method does not have any special behavior for index property
   inquiries, the <code class="structfield">amproperty</code> field in
   its <code class="structname">IndexAmRoutine</code> struct can be set to NULL.
   Otherwise, the <code class="function">amproperty</code> method will be called with
   <em class="parameter"><code>index_oid</code></em> and <em class="parameter"><code>attno</code></em> both zero for
   <code class="function">pg_indexam_has_property</code> calls,
   or with <em class="parameter"><code>index_oid</code></em> valid and <em class="parameter"><code>attno</code></em> zero for
   <code class="function">pg_index_has_property</code> calls,
   or with <em class="parameter"><code>index_oid</code></em> valid and <em class="parameter"><code>attno</code></em> greater than
   zero for <code class="function">pg_index_column_has_property</code> calls.
   <em class="parameter"><code>prop</code></em> is an enum value identifying the property being tested,
   while <em class="parameter"><code>propname</code></em> is the original property name string.
   If the core code does not recognize the property name
   then <em class="parameter"><code>prop</code></em> is <code class="literal">AMPROP_UNKNOWN</code>.
   Access methods can define custom property names by
   checking <em class="parameter"><code>propname</code></em> for a match (use <code class="function">pg_strcasecmp</code>
   to match, for consistency with the core code); for names known to the core
   code, it's better to inspect <em class="parameter"><code>prop</code></em>.
   If the <code class="structfield">amproperty</code> method returns <code class="literal">true</code> then
   it has determined the property test result: it must set <code class="literal">*res</code>
   to the boolean value to return, or set <code class="literal">*isnull</code>
   to <code class="literal">true</code> to return a NULL.  (Both of the referenced variables
   are initialized to <code class="literal">false</code> before the call.)
   If the <code class="structfield">amproperty</code> method returns <code class="literal">false</code> then
   the core code will proceed with its normal logic for determining the
   property test result.
  </p><p>
   Access methods that support ordering operators should
   implement <code class="literal">AMPROP_DISTANCE_ORDERABLE</code> property testing, as the
   core code does not know how to do that and will return NULL.  It may
   also be advantageous to implement <code class="literal">AMPROP_RETURNABLE</code> testing,
   if that can be done more cheaply than by opening the index and calling
   <code class="structfield">amcanreturn</code>, which is the core code's default behavior.
   The default behavior should be satisfactory for all other standard
   properties.
  </p><p>
</p><pre class="programlisting">
char *
ambuildphasename (int64 phasenum);
</pre><p>
   Return the textual name of the given build phase number.
   The phase numbers are those reported during an index build via the
   <code class="function">pgstat_progress_update_param</code> interface.
   The phase names are then exposed in the
   <code class="structname">pg_stat_progress_create_index</code> view.
  </p><p>
</p><pre class="programlisting">
bool
amvalidate (Oid opclassoid);
</pre><p>
   Validate the catalog entries for the specified operator class, so far as
   the access method can reasonably do that.  For example, this might include
   testing that all required support functions are provided.
   The <code class="function">amvalidate</code> function must return false if the opclass is
   invalid.  Problems should be reported with <code class="function">ereport</code> messages.
  </p><p>
   The purpose of an index, of course, is to support scans for tuples matching
   an indexable <code class="literal">WHERE</code> condition, often called a
   <em class="firstterm">qualifier</em> or <em class="firstterm">scan key</em>.  The semantics of
   index scanning are described more fully in <a class="xref" href="index-scanning.html" title="61.3. Index Scanning">Section 61.3</a>,
   below.  An index access method can support <span class="quote">“<span class="quote">plain</span>”</span> index scans,
   <span class="quote">“<span class="quote">bitmap</span>”</span> index scans, or both.  The scan-related functions that an
   index access method must or may provide are:
  </p><p>
</p><pre class="programlisting">
IndexScanDesc
ambeginscan (Relation indexRelation,
             int nkeys,
             int norderbys);
</pre><p>
   Prepare for an index scan.  The <code class="literal">nkeys</code> and <code class="literal">norderbys</code>
   parameters indicate the number of quals and ordering operators that will be
   used in the scan; these may be useful for space allocation purposes.
   Note that the actual values of the scan keys aren't provided yet.
   The result must be a palloc'd struct.
   For implementation reasons the index access method
   <span class="emphasis"><em>must</em></span> create this struct by calling
   <code class="function">RelationGetIndexScan()</code>.  In most cases
   <code class="function">ambeginscan</code> does little beyond making that call and perhaps
   acquiring locks;
   the interesting parts of index-scan startup are in <code class="function">amrescan</code>.
  </p><p>
</p><pre class="programlisting">
void
amrescan (IndexScanDesc scan,
          ScanKey keys,
          int nkeys,
          ScanKey orderbys,
          int norderbys);
</pre><p>
   Start or restart an index scan, possibly with new scan keys.  (To restart
   using previously-passed keys, NULL is passed for <code class="literal">keys</code> and/or
   <code class="literal">orderbys</code>.)  Note that it is not allowed for
   the number of keys or order-by operators to be larger than
   what was passed to <code class="function">ambeginscan</code>.  In practice the restart
   feature is used when a new outer tuple is selected by a nested-loop join
   and so a new key comparison value is needed, but the scan key structure
   remains the same.
  </p><p>
</p><pre class="programlisting">
boolean
amgettuple (IndexScanDesc scan,
            ScanDirection direction);
</pre><p>
   Fetch the next tuple in the given scan, moving in the given
   direction (forward or backward in the index).  Returns true if a tuple was
   obtained, false if no matching tuples remain.  In the true case the tuple
   TID is stored into the <code class="literal">scan</code> structure.  Note that
   <span class="quote">“<span class="quote">success</span>”</span> means only that the index contains an entry that matches
   the scan keys, not that the tuple necessarily still exists in the heap or
   will pass the caller's snapshot test.  On success, <code class="function">amgettuple</code>
   must also set <code class="literal">scan-&gt;xs_recheck</code> to true or false.
   False means it is certain that the index entry matches the scan keys.
   true means this is not certain, and the conditions represented by the
   scan keys must be rechecked against the heap tuple after fetching it.
   This provision supports <span class="quote">“<span class="quote">lossy</span>”</span> index operators.
   Note that rechecking will extend only to the scan conditions; a partial
   index predicate (if any) is never rechecked by <code class="function">amgettuple</code>
   callers.
  </p><p>
   If the index supports <a class="link" href="indexes-index-only-scans.html" title="11.9. Index-Only Scans and Covering Indexes">index-only
   scans</a> (i.e., <code class="function">amcanreturn</code> returns true for it),
   then on success the AM must also check <code class="literal">scan-&gt;xs_want_itup</code>,
   and if that is true it must return the originally indexed data for the
   index entry.  The data can be returned in the form of an
   <code class="structname">IndexTuple</code> pointer stored at <code class="literal">scan-&gt;xs_itup</code>,
   with tuple descriptor <code class="literal">scan-&gt;xs_itupdesc</code>; or in the form of
   a <code class="structname">HeapTuple</code> pointer stored at <code class="literal">scan-&gt;xs_hitup</code>,
   with tuple descriptor <code class="literal">scan-&gt;xs_hitupdesc</code>.  (The latter
   format should be used when reconstructing data that might possibly not fit
   into an <code class="structname">IndexTuple</code>.)  In either case,
   management of the data referenced by the pointer is the access method's
   responsibility.  The data must remain good at least until the next
   <code class="function">amgettuple</code>, <code class="function">amrescan</code>, or <code class="function">amendscan</code>
   call for the scan.
  </p><p>
   The <code class="function">amgettuple</code> function need only be provided if the access
   method supports <span class="quote">“<span class="quote">plain</span>”</span> index scans.  If it doesn't, the
   <code class="structfield">amgettuple</code> field in its <code class="structname">IndexAmRoutine</code>
   struct must be set to NULL.
  </p><p>
</p><pre class="programlisting">
int64
amgetbitmap (IndexScanDesc scan,
             TIDBitmap *tbm);
</pre><p>
   Fetch all tuples in the given scan and add them to the caller-supplied
   <code class="type">TIDBitmap</code> (that is, OR the set of tuple IDs into whatever set is already
   in the bitmap).  The number of tuples fetched is returned (this might be
   just an approximate count, for instance some AMs do not detect duplicates).
   While inserting tuple IDs into the bitmap, <code class="function">amgetbitmap</code> can
   indicate that rechecking of the scan conditions is required for specific
   tuple IDs.  This is analogous to the <code class="literal">xs_recheck</code> output parameter
   of <code class="function">amgettuple</code>.  Note: in the current implementation, support
   for this feature is conflated with support for lossy storage of the bitmap
   itself, and therefore callers recheck both the scan conditions and the
   partial index predicate (if any) for recheckable tuples.  That might not
   always be true, however.
   <code class="function">amgetbitmap</code> and
   <code class="function">amgettuple</code> cannot be used in the same index scan; there
   are other restrictions too when using <code class="function">amgetbitmap</code>, as explained
   in <a class="xref" href="index-scanning.html" title="61.3. Index Scanning">Section 61.3</a>.
  </p><p>
   The <code class="function">amgetbitmap</code> function need only be provided if the access
   method supports <span class="quote">“<span class="quote">bitmap</span>”</span> index scans.  If it doesn't, the
   <code class="structfield">amgetbitmap</code> field in its <code class="structname">IndexAmRoutine</code>
   struct must be set to NULL.
  </p><p>
</p><pre class="programlisting">
void
amendscan (IndexScanDesc scan);
</pre><p>
   End a scan and release resources.  The <code class="literal">scan</code> struct itself
   should not be freed, but any locks or pins taken internally by the
   access method must be released, as well as any other memory allocated
   by <code class="function">ambeginscan</code> and other scan-related functions.
  </p><p>
</p><pre class="programlisting">
void
ammarkpos (IndexScanDesc scan);
</pre><p>
   Mark current scan position.  The access method need only support one
   remembered scan position per scan.
  </p><p>
   The <code class="function">ammarkpos</code> function need only be provided if the access
   method supports ordered scans.  If it doesn't,
   the <code class="structfield">ammarkpos</code> field in its <code class="structname">IndexAmRoutine</code>
   struct may be set to NULL.
  </p><p>
</p><pre class="programlisting">
void
amrestrpos (IndexScanDesc scan);
</pre><p>
   Restore the scan to the most recently marked position.
  </p><p>
   The <code class="function">amrestrpos</code> function need only be provided if the access
   method supports ordered scans.  If it doesn't,
   the <code class="structfield">amrestrpos</code> field in its <code class="structname">IndexAmRoutine</code>
   struct may be set to NULL.
  </p><p>
   In addition to supporting ordinary index scans, some types of index
   may wish to support <em class="firstterm">parallel index scans</em>, which allow
   multiple backends to cooperate in performing an index scan.  The
   index access method should arrange things so that each cooperating
   process returns a subset of the tuples that would be performed by
   an ordinary, non-parallel index scan, but in such a way that the
   union of those subsets is equal to the set of tuples that would be
   returned by an ordinary, non-parallel index scan.  Furthermore, while
   there need not be any global ordering of tuples returned by a parallel
   scan, the ordering of that subset of tuples returned within each
   cooperating backend must match the requested ordering.  The following
   functions may be implemented to support parallel index scans:
  </p><p>
</p><pre class="programlisting">
Size
amestimateparallelscan (void);
</pre><p>
   Estimate and return the number of bytes of dynamic shared memory which
   the access method will be needed to perform a parallel scan.  (This number
   is in addition to, not in lieu of, the amount of space needed for
   AM-independent data in <code class="structname">ParallelIndexScanDescData</code>.)
  </p><p>
   It is not necessary to implement this function for access methods which
   do not support parallel scans or for which the number of additional bytes
   of storage required is zero.
  </p><p>
</p><pre class="programlisting">
void
aminitparallelscan (void *target);
</pre><p>
   This function will be called to initialize dynamic shared memory at the
   beginning of a parallel scan.  <em class="parameter"><code>target</code></em> will point to at least
   the number of bytes previously returned by
   <code class="function">amestimateparallelscan</code>, and this function may use that
   amount of space to store whatever data it wishes.
  </p><p>
   It is not necessary to implement this function for access methods which
   do not support parallel scans or in cases where the shared memory space
   required needs no initialization.
  </p><p>
</p><pre class="programlisting">
void
amparallelrescan (IndexScanDesc scan);
</pre><p>
   This function, if implemented, will be called when a parallel index scan
   must be restarted.  It should reset any shared state set up by
   <code class="function">aminitparallelscan</code> such that the scan will be restarted from
   the beginning.
  </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="index-api.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="indexam.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="index-scanning.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">61.1. Basic API Structure for Indexes </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> 61.3. Index Scanning</td></tr></table></div></body></html>