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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>62.2. 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 Vsnapshot" /><link rel="prev" href="index-api.html" title="62.1. Basic API Structure for Indexes" /><link rel="next" href="index-scanning.html" title="62.3. Index Scanning" /></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">62.2. Index Access Method Functions</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="index-api.html" title="62.1. Basic API Structure for Indexes">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="indexam.html" title="Chapter 62. Index Access Method Interface Definition">Up</a></td><th width="60%" align="center">Chapter 62. Index Access Method Interface Definition</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="index-scanning.html" title="62.3. 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">62.2. 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,
+ bool indexUnchanged,
+ 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="62.5. Index Uniqueness Checks">Section 62.5</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 <code class="literal">indexUnchanged</code> Boolean value gives a hint
+ about the nature of the tuple to be indexed. When it is true,
+ the tuple is a duplicate of some existing tuple in the index. The
+ new tuple is a logically unchanged successor MVCC tuple version. This
+ happens when an <code class="command">UPDATE</code> takes place that does not
+ modify any columns covered by the index, but nevertheless requires a
+ new version in the index. The index AM may use this hint to decide
+ to apply bottom-up index deletion in parts of the index where many
+ versions of the same logical row accumulate. Note that updating a
+ non-key column does not affect the value of
+ <code class="literal">indexUnchanged</code>.
+ </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 an 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>
+ <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 column's original indexed value.
+ The attribute number is 1-based, i.e., the first column's attno is 1.
+ Returns true if supported, else false.
+ This function should always return true for included columns
+ (if those are supported), since there's little point in an included
+ column that can't be retrieved.
+ 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="62.6. Index Cost Estimation Functions">Section 62.6</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="function">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, typically at <code class="literal">INFO</code> level.
+ </p><p>
+</p><pre class="programlisting">
+void
+amadjustmembers (Oid opfamilyoid,
+ Oid opclassoid,
+ List *operators,
+ List *functions);
+</pre><p>
+ Validate proposed new operator and function members of an operator family,
+ so far as the access method can reasonably do that, and set their
+ dependency types if the default is not satisfactory. This is called
+ during <code class="command">CREATE OPERATOR CLASS</code> and during
+ <code class="command">ALTER OPERATOR FAMILY ADD</code>; in the latter
+ case <em class="parameter"><code>opclassoid</code></em> is <code class="literal">InvalidOid</code>.
+ The <code class="type">List</code> arguments are lists
+ of <code class="structname">OpFamilyMember</code> structs, as defined
+ in <code class="filename">amapi.h</code>.
+
+ Tests done by this function will typically be a subset of those
+ performed by <code class="function">amvalidate</code>,
+ since <code class="function">amadjustmembers</code> cannot assume that it is
+ seeing a complete set of members. For example, it would be reasonable
+ to check the signature of a support function, but not to check whether
+ all required support functions are provided. Any problems can be
+ reported by throwing an error.
+
+ The dependency-related fields of
+ the <code class="structname">OpFamilyMember</code> structs are initialized by
+ the core code to create hard dependencies on the opclass if this
+ is <code class="command">CREATE OPERATOR CLASS</code>, or soft dependencies on the
+ opfamily if this is <code class="command">ALTER OPERATOR FAMILY ADD</code>.
+ <code class="function">amadjustmembers</code> can adjust these fields if some other
+ behavior is more appropriate. For example, GIN, GiST, and SP-GiST
+ always set operator members to have soft dependencies on the opfamily,
+ since the connection between an operator and an opclass is relatively
+ weak in these index types; so it is reasonable to allow operator members
+ to be added and removed freely. Optional support functions are typically
+ also given soft dependencies, so that they can be removed if necessary.
+ </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="62.3. Index Scanning">Section 62.3</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">
+bool
+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 any
+ of its columns),
+ 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. Columns for which <code class="function">amcanreturn</code> returns
+ false can be returned as nulls.
+ 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="62.3. Index Scanning">Section 62.3</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 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="index-api.html" title="62.1. Basic API Structure for Indexes">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="indexam.html" title="Chapter 62. Index Access Method Interface Definition">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="index-scanning.html" title="62.3. Index Scanning">Next</a></td></tr><tr><td width="40%" align="left" valign="top">62.1. Basic API Structure for Indexes </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 14.5 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 62.3. Index Scanning</td></tr></table></div></body></html> \ No newline at end of file