1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
|
<?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>64.2. 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="64.1. Basic API Structure for Indexes" /><link rel="next" href="index-scanning.html" title="64.3. Index Scanning" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">64.2. Index Access Method Functions</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="index-api.html" title="64.1. Basic API Structure for Indexes">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="indexam.html" title="Chapter 64. Index Access Method Interface Definition">Up</a></td><th width="60%" align="center">Chapter 64. Index Access Method Interface Definition</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 15.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="index-scanning.html" title="64.3. Index Scanning">Next</a></td></tr></table><hr /></div><div class="sect1" id="INDEX-FUNCTIONS"><div class="titlepage"><div><div><h2 class="title" style="clear: both">64.2. 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="64.5. Index Uniqueness Checks">Section 64.5</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 or a column that only appears in a partial index predicate does not
affect the value of <code class="literal">indexUnchanged</code>. The core code
determines each tuple's <code class="literal">indexUnchanged</code> value using a low
overhead approach that allows both false positives and false negatives.
Index AMs must not treat <code class="literal">indexUnchanged</code> as an
authoritative source of information about tuple visibility or versioning.
</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->ii_Context</code> and store a pointer to the
data in <code class="literal">indexInfo->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->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="64.6. Index Cost Estimation Functions">Section 64.6</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="64.3. Index Scanning">Section 64.3</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->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->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->xs_itup</code>,
with tuple descriptor <code class="literal">scan->xs_itupdesc</code>; or in the form of
a <code class="structname">HeapTuple</code> pointer stored at <code class="literal">scan->xs_hitup</code>,
with tuple descriptor <code class="literal">scan->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="64.3. Index Scanning">Section 64.3</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" title="64.1. Basic API Structure for Indexes">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="indexam.html" title="Chapter 64. Index Access Method Interface Definition">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="index-scanning.html" title="64.3. Index Scanning">Next</a></td></tr><tr><td width="40%" align="left" valign="top">64.1. Basic API Structure for Indexes </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.5 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 64.3. Index Scanning</td></tr></table></div></body></html>
|
