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
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
|
<?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>20.7. Query Planning</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="runtime-config-replication.html" title="20.6. Replication" /><link rel="next" href="runtime-config-logging.html" title="20.8. Error Reporting and Logging" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">20.7. Query Planning</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="runtime-config-replication.html" title="20.6. Replication">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="runtime-config.html" title="Chapter 20. Server Configuration">Up</a></td><th width="60%" align="center">Chapter 20. Server Configuration</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 15.7 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="runtime-config-logging.html" title="20.8. Error Reporting and Logging">Next</a></td></tr></table><hr /></div><div class="sect1" id="RUNTIME-CONFIG-QUERY"><div class="titlepage"><div><div><h2 class="title" style="clear: both">20.7. Query Planning</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="runtime-config-query.html#RUNTIME-CONFIG-QUERY-ENABLE">20.7.1. Planner Method Configuration</a></span></dt><dt><span class="sect2"><a href="runtime-config-query.html#RUNTIME-CONFIG-QUERY-CONSTANTS">20.7.2. Planner Cost Constants</a></span></dt><dt><span class="sect2"><a href="runtime-config-query.html#RUNTIME-CONFIG-QUERY-GEQO">20.7.3. Genetic Query Optimizer</a></span></dt><dt><span class="sect2"><a href="runtime-config-query.html#RUNTIME-CONFIG-QUERY-OTHER">20.7.4. Other Planner Options</a></span></dt></dl></div><div class="sect2" id="RUNTIME-CONFIG-QUERY-ENABLE"><div class="titlepage"><div><div><h3 class="title">20.7.1. Planner Method Configuration</h3></div></div></div><p>
These configuration parameters provide a crude method of
influencing the query plans chosen by the query optimizer. If
the default plan chosen by the optimizer for a particular query
is not optimal, a <span class="emphasis"><em>temporary</em></span> solution is to use one
of these configuration parameters to force the optimizer to
choose a different plan.
Better ways to improve the quality of the
plans chosen by the optimizer include adjusting the planner cost
constants (see <a class="xref" href="runtime-config-query.html#RUNTIME-CONFIG-QUERY-CONSTANTS" title="20.7.2. Planner Cost Constants">Section 20.7.2</a>),
running <a class="link" href="sql-analyze.html" title="ANALYZE"><code class="command">ANALYZE</code></a> manually, increasing
the value of the <a class="xref" href="runtime-config-query.html#GUC-DEFAULT-STATISTICS-TARGET">default_statistics_target</a> configuration parameter,
and increasing the amount of statistics collected for
specific columns using <code class="command">ALTER TABLE SET
STATISTICS</code>.
</p><div class="variablelist"><dl class="variablelist"><dt id="GUC-ENABLE-ASYNC-APPEND"><span class="term"><code class="varname">enable_async_append</code> (<code class="type">boolean</code>)
<a id="id-1.6.7.10.2.3.1.1.3" class="indexterm"></a>
</span></dt><dd><p>
Enables or disables the query planner's use of async-aware
append plan types. The default is <code class="literal">on</code>.
</p></dd><dt id="GUC-ENABLE-BITMAPSCAN"><span class="term"><code class="varname">enable_bitmapscan</code> (<code class="type">boolean</code>)
<a id="id-1.6.7.10.2.3.2.1.3" class="indexterm"></a>
<a id="id-1.6.7.10.2.3.2.1.4" class="indexterm"></a>
</span></dt><dd><p>
Enables or disables the query planner's use of bitmap-scan plan
types. The default is <code class="literal">on</code>.
</p></dd><dt id="GUC-ENABLE-GATHERMERGE"><span class="term"><code class="varname">enable_gathermerge</code> (<code class="type">boolean</code>)
<a id="id-1.6.7.10.2.3.3.1.3" class="indexterm"></a>
</span></dt><dd><p>
Enables or disables the query planner's use of gather
merge plan types. The default is <code class="literal">on</code>.
</p></dd><dt id="GUC-ENABLE-HASHAGG"><span class="term"><code class="varname">enable_hashagg</code> (<code class="type">boolean</code>)
<a id="id-1.6.7.10.2.3.4.1.3" class="indexterm"></a>
</span></dt><dd><p>
Enables or disables the query planner's use of hashed
aggregation plan types. The default is <code class="literal">on</code>.
</p></dd><dt id="GUC-ENABLE-HASHJOIN"><span class="term"><code class="varname">enable_hashjoin</code> (<code class="type">boolean</code>)
<a id="id-1.6.7.10.2.3.5.1.3" class="indexterm"></a>
</span></dt><dd><p>
Enables or disables the query planner's use of hash-join plan
types. The default is <code class="literal">on</code>.
</p></dd><dt id="GUC-ENABLE-INCREMENTAL-SORT"><span class="term"><code class="varname">enable_incremental_sort</code> (<code class="type">boolean</code>)
<a id="id-1.6.7.10.2.3.6.1.3" class="indexterm"></a>
</span></dt><dd><p>
Enables or disables the query planner's use of incremental sort steps.
The default is <code class="literal">on</code>.
</p></dd><dt id="GUC-ENABLE-INDEXSCAN"><span class="term"><code class="varname">enable_indexscan</code> (<code class="type">boolean</code>)
<a id="id-1.6.7.10.2.3.7.1.3" class="indexterm"></a>
<a id="id-1.6.7.10.2.3.7.1.4" class="indexterm"></a>
</span></dt><dd><p>
Enables or disables the query planner's use of index-scan plan
types. The default is <code class="literal">on</code>.
</p></dd><dt id="GUC-ENABLE-INDEXONLYSCAN"><span class="term"><code class="varname">enable_indexonlyscan</code> (<code class="type">boolean</code>)
<a id="id-1.6.7.10.2.3.8.1.3" class="indexterm"></a>
</span></dt><dd><p>
Enables or disables the query planner's use of index-only-scan plan
types (see <a class="xref" href="indexes-index-only-scans.html" title="11.9. Index-Only Scans and Covering Indexes">Section 11.9</a>).
The default is <code class="literal">on</code>.
</p></dd><dt id="GUC-ENABLE-MATERIAL"><span class="term"><code class="varname">enable_material</code> (<code class="type">boolean</code>)
<a id="id-1.6.7.10.2.3.9.1.3" class="indexterm"></a>
</span></dt><dd><p>
Enables or disables the query planner's use of materialization.
It is impossible to suppress materialization entirely,
but turning this variable off prevents the planner from inserting
materialize nodes except in cases where it is required for correctness.
The default is <code class="literal">on</code>.
</p></dd><dt id="GUC-ENABLE-MEMOIZE"><span class="term"><code class="varname">enable_memoize</code> (<code class="type">boolean</code>)
<a id="id-1.6.7.10.2.3.10.1.3" class="indexterm"></a>
</span></dt><dd><p>
Enables or disables the query planner's use of memoize plans for
caching results from parameterized scans inside nested-loop joins.
This plan type allows scans to the underlying plans to be skipped when
the results for the current parameters are already in the cache. Less
commonly looked up results may be evicted from the cache when more
space is required for new entries. The default is
<code class="literal">on</code>.
</p></dd><dt id="GUC-ENABLE-MERGEJOIN"><span class="term"><code class="varname">enable_mergejoin</code> (<code class="type">boolean</code>)
<a id="id-1.6.7.10.2.3.11.1.3" class="indexterm"></a>
</span></dt><dd><p>
Enables or disables the query planner's use of merge-join plan
types. The default is <code class="literal">on</code>.
</p></dd><dt id="GUC-ENABLE-NESTLOOP"><span class="term"><code class="varname">enable_nestloop</code> (<code class="type">boolean</code>)
<a id="id-1.6.7.10.2.3.12.1.3" class="indexterm"></a>
</span></dt><dd><p>
Enables or disables the query planner's use of nested-loop join
plans. It is impossible to suppress nested-loop joins entirely,
but turning this variable off discourages the planner from using
one if there are other methods available. The default is
<code class="literal">on</code>.
</p></dd><dt id="GUC-ENABLE-PARALLEL-APPEND"><span class="term"><code class="varname">enable_parallel_append</code> (<code class="type">boolean</code>)
<a id="id-1.6.7.10.2.3.13.1.3" class="indexterm"></a>
</span></dt><dd><p>
Enables or disables the query planner's use of parallel-aware
append plan types. The default is <code class="literal">on</code>.
</p></dd><dt id="GUC-ENABLE-PARALLEL-HASH"><span class="term"><code class="varname">enable_parallel_hash</code> (<code class="type">boolean</code>)
<a id="id-1.6.7.10.2.3.14.1.3" class="indexterm"></a>
</span></dt><dd><p>
Enables or disables the query planner's use of hash-join plan
types with parallel hash. Has no effect if hash-join plans are not
also enabled. The default is <code class="literal">on</code>.
</p></dd><dt id="GUC-ENABLE-PARTITION-PRUNING"><span class="term"><code class="varname">enable_partition_pruning</code> (<code class="type">boolean</code>)
<a id="id-1.6.7.10.2.3.15.1.3" class="indexterm"></a>
</span></dt><dd><p>
Enables or disables the query planner's ability to eliminate a
partitioned table's partitions from query plans. This also controls
the planner's ability to generate query plans which allow the query
executor to remove (ignore) partitions during query execution. The
default is <code class="literal">on</code>.
See <a class="xref" href="ddl-partitioning.html#DDL-PARTITION-PRUNING" title="5.11.4. Partition Pruning">Section 5.11.4</a> for details.
</p></dd><dt id="GUC-ENABLE-PARTITIONWISE-JOIN"><span class="term"><code class="varname">enable_partitionwise_join</code> (<code class="type">boolean</code>)
<a id="id-1.6.7.10.2.3.16.1.3" class="indexterm"></a>
</span></dt><dd><p>
Enables or disables the query planner's use of partitionwise join,
which allows a join between partitioned tables to be performed by
joining the matching partitions. Partitionwise join currently applies
only when the join conditions include all the partition keys, which
must be of the same data type and have one-to-one matching sets of
child partitions. Because partitionwise join planning can use
significantly more CPU time and memory during planning, the default is
<code class="literal">off</code>.
</p></dd><dt id="GUC-ENABLE-PARTITIONWISE-AGGREGATE"><span class="term"><code class="varname">enable_partitionwise_aggregate</code> (<code class="type">boolean</code>)
<a id="id-1.6.7.10.2.3.17.1.3" class="indexterm"></a>
</span></dt><dd><p>
Enables or disables the query planner's use of partitionwise grouping
or aggregation, which allows grouping or aggregation on partitioned
tables to be performed separately for each partition. If the
<code class="literal">GROUP BY</code> clause does not include the partition
keys, only partial aggregation can be performed on a per-partition
basis, and finalization must be performed later. Because
partitionwise grouping or aggregation can use significantly more CPU
time and memory during planning, the default is
<code class="literal">off</code>.
</p></dd><dt id="GUC-ENABLE-SEQSCAN"><span class="term"><code class="varname">enable_seqscan</code> (<code class="type">boolean</code>)
<a id="id-1.6.7.10.2.3.18.1.3" class="indexterm"></a>
<a id="id-1.6.7.10.2.3.18.1.4" class="indexterm"></a>
</span></dt><dd><p>
Enables or disables the query planner's use of sequential scan
plan types. It is impossible to suppress sequential scans
entirely, but turning this variable off discourages the planner
from using one if there are other methods available. The
default is <code class="literal">on</code>.
</p></dd><dt id="GUC-ENABLE-SORT"><span class="term"><code class="varname">enable_sort</code> (<code class="type">boolean</code>)
<a id="id-1.6.7.10.2.3.19.1.3" class="indexterm"></a>
</span></dt><dd><p>
Enables or disables the query planner's use of explicit sort
steps. It is impossible to suppress explicit sorts entirely,
but turning this variable off discourages the planner from
using one if there are other methods available. The default
is <code class="literal">on</code>.
</p></dd><dt id="GUC-ENABLE-TIDSCAN"><span class="term"><code class="varname">enable_tidscan</code> (<code class="type">boolean</code>)
<a id="id-1.6.7.10.2.3.20.1.3" class="indexterm"></a>
</span></dt><dd><p>
Enables or disables the query planner's use of <acronym class="acronym">TID</acronym>
scan plan types. The default is <code class="literal">on</code>.
</p></dd></dl></div></div><div class="sect2" id="RUNTIME-CONFIG-QUERY-CONSTANTS"><div class="titlepage"><div><div><h3 class="title">20.7.2. Planner Cost Constants</h3></div></div></div><p>
The <em class="firstterm">cost</em> variables described in this section are measured
on an arbitrary scale. Only their relative values matter, hence
scaling them all up or down by the same factor will result in no change
in the planner's choices. By default, these cost variables are based on
the cost of sequential page fetches; that is,
<code class="varname">seq_page_cost</code> is conventionally set to <code class="literal">1.0</code>
and the other cost variables are set with reference to that. But
you can use a different scale if you prefer, such as actual execution
times in milliseconds on a particular machine.
</p><div class="note"><h3 class="title">Note</h3><p>
Unfortunately, there is no well-defined method for determining ideal
values for the cost variables. They are best treated as averages over
the entire mix of queries that a particular installation will receive. This
means that changing them on the basis of just a few experiments is very
risky.
</p></div><div class="variablelist"><dl class="variablelist"><dt id="GUC-SEQ-PAGE-COST"><span class="term"><code class="varname">seq_page_cost</code> (<code class="type">floating point</code>)
<a id="id-1.6.7.10.3.4.1.1.3" class="indexterm"></a>
</span></dt><dd><p>
Sets the planner's estimate of the cost of a disk page fetch
that is part of a series of sequential fetches. The default is 1.0.
This value can be overridden for tables and indexes in a particular
tablespace by setting the tablespace parameter of the same name
(see <a class="xref" href="sql-altertablespace.html" title="ALTER TABLESPACE"><span class="refentrytitle">ALTER TABLESPACE</span></a>).
</p></dd><dt id="GUC-RANDOM-PAGE-COST"><span class="term"><code class="varname">random_page_cost</code> (<code class="type">floating point</code>)
<a id="id-1.6.7.10.3.4.2.1.3" class="indexterm"></a>
</span></dt><dd><p>
Sets the planner's estimate of the cost of a
non-sequentially-fetched disk page. The default is 4.0.
This value can be overridden for tables and indexes in a particular
tablespace by setting the tablespace parameter of the same name
(see <a class="xref" href="sql-altertablespace.html" title="ALTER TABLESPACE"><span class="refentrytitle">ALTER TABLESPACE</span></a>).
</p><p>
Reducing this value relative to <code class="varname">seq_page_cost</code>
will cause the system to prefer index scans; raising it will
make index scans look relatively more expensive. You can raise
or lower both values together to change the importance of disk I/O
costs relative to CPU costs, which are described by the following
parameters.
</p><p>
Random access to mechanical disk storage is normally much more expensive
than four times sequential access. However, a lower default is used
(4.0) because the majority of random accesses to disk, such as indexed
reads, are assumed to be in cache. The default value can be thought of
as modeling random access as 40 times slower than sequential, while
expecting 90% of random reads to be cached.
</p><p>
If you believe a 90% cache rate is an incorrect assumption
for your workload, you can increase random_page_cost to better
reflect the true cost of random storage reads. Correspondingly,
if your data is likely to be completely in cache, such as when
the database is smaller than the total server memory, decreasing
random_page_cost can be appropriate. Storage that has a low random
read cost relative to sequential, e.g., solid-state drives, might
also be better modeled with a lower value for random_page_cost,
e.g., <code class="literal">1.1</code>.
</p><div class="tip"><h3 class="title">Tip</h3><p>
Although the system will let you set <code class="varname">random_page_cost</code> to
less than <code class="varname">seq_page_cost</code>, it is not physically sensible
to do so. However, setting them equal makes sense if the database
is entirely cached in RAM, since in that case there is no penalty
for touching pages out of sequence. Also, in a heavily-cached
database you should lower both values relative to the CPU parameters,
since the cost of fetching a page already in RAM is much smaller
than it would normally be.
</p></div></dd><dt id="GUC-CPU-TUPLE-COST"><span class="term"><code class="varname">cpu_tuple_cost</code> (<code class="type">floating point</code>)
<a id="id-1.6.7.10.3.4.3.1.3" class="indexterm"></a>
</span></dt><dd><p>
Sets the planner's estimate of the cost of processing
each row during a query.
The default is 0.01.
</p></dd><dt id="GUC-CPU-INDEX-TUPLE-COST"><span class="term"><code class="varname">cpu_index_tuple_cost</code> (<code class="type">floating point</code>)
<a id="id-1.6.7.10.3.4.4.1.3" class="indexterm"></a>
</span></dt><dd><p>
Sets the planner's estimate of the cost of processing
each index entry during an index scan.
The default is 0.005.
</p></dd><dt id="GUC-CPU-OPERATOR-COST"><span class="term"><code class="varname">cpu_operator_cost</code> (<code class="type">floating point</code>)
<a id="id-1.6.7.10.3.4.5.1.3" class="indexterm"></a>
</span></dt><dd><p>
Sets the planner's estimate of the cost of processing each
operator or function executed during a query.
The default is 0.0025.
</p></dd><dt id="GUC-PARALLEL-SETUP-COST"><span class="term"><code class="varname">parallel_setup_cost</code> (<code class="type">floating point</code>)
<a id="id-1.6.7.10.3.4.6.1.3" class="indexterm"></a>
</span></dt><dd><p>
Sets the planner's estimate of the cost of launching parallel worker
processes.
The default is 1000.
</p></dd><dt id="GUC-PARALLEL-TUPLE-COST"><span class="term"><code class="varname">parallel_tuple_cost</code> (<code class="type">floating point</code>)
<a id="id-1.6.7.10.3.4.7.1.3" class="indexterm"></a>
</span></dt><dd><p>
Sets the planner's estimate of the cost of transferring one tuple
from a parallel worker process to another process.
The default is 0.1.
</p></dd><dt id="GUC-MIN-PARALLEL-TABLE-SCAN-SIZE"><span class="term"><code class="varname">min_parallel_table_scan_size</code> (<code class="type">integer</code>)
<a id="id-1.6.7.10.3.4.8.1.3" class="indexterm"></a>
</span></dt><dd><p>
Sets the minimum amount of table data that must be scanned in order
for a parallel scan to be considered. For a parallel sequential scan,
the amount of table data scanned is always equal to the size of the
table, but when indexes are used the amount of table data
scanned will normally be less.
If this value is specified without units, it is taken as blocks,
that is <code class="symbol">BLCKSZ</code> bytes, typically 8kB.
The default is 8 megabytes (<code class="literal">8MB</code>).
</p></dd><dt id="GUC-MIN-PARALLEL-INDEX-SCAN-SIZE"><span class="term"><code class="varname">min_parallel_index_scan_size</code> (<code class="type">integer</code>)
<a id="id-1.6.7.10.3.4.9.1.3" class="indexterm"></a>
</span></dt><dd><p>
Sets the minimum amount of index data that must be scanned in order
for a parallel scan to be considered. Note that a parallel index scan
typically won't touch the entire index; it is the number of pages
which the planner believes will actually be touched by the scan which
is relevant. This parameter is also used to decide whether a
particular index can participate in a parallel vacuum. See
<a class="xref" href="sql-vacuum.html" title="VACUUM"><span class="refentrytitle">VACUUM</span></a>.
If this value is specified without units, it is taken as blocks,
that is <code class="symbol">BLCKSZ</code> bytes, typically 8kB.
The default is 512 kilobytes (<code class="literal">512kB</code>).
</p></dd><dt id="GUC-EFFECTIVE-CACHE-SIZE"><span class="term"><code class="varname">effective_cache_size</code> (<code class="type">integer</code>)
<a id="id-1.6.7.10.3.4.10.1.3" class="indexterm"></a>
</span></dt><dd><p>
Sets the planner's assumption about the effective size of the
disk cache that is available to a single query. This is
factored into estimates of the cost of using an index; a
higher value makes it more likely index scans will be used, a
lower value makes it more likely sequential scans will be
used. When setting this parameter you should consider both
<span class="productname">PostgreSQL</span>'s shared buffers and the
portion of the kernel's disk cache that will be used for
<span class="productname">PostgreSQL</span> data files, though some
data might exist in both places. Also, take
into account the expected number of concurrent queries on different
tables, since they will have to share the available
space. This parameter has no effect on the size of shared
memory allocated by <span class="productname">PostgreSQL</span>, nor
does it reserve kernel disk cache; it is used only for estimation
purposes. The system also does not assume data remains in
the disk cache between queries.
If this value is specified without units, it is taken as blocks,
that is <code class="symbol">BLCKSZ</code> bytes, typically 8kB.
The default is 4 gigabytes (<code class="literal">4GB</code>).
(If <code class="symbol">BLCKSZ</code> is not 8kB, the default value scales
proportionally to it.)
</p></dd><dt id="GUC-JIT-ABOVE-COST"><span class="term"><code class="varname">jit_above_cost</code> (<code class="type">floating point</code>)
<a id="id-1.6.7.10.3.4.11.1.3" class="indexterm"></a>
</span></dt><dd><p>
Sets the query cost above which JIT compilation is activated, if
enabled (see <a class="xref" href="jit.html" title="Chapter 32. Just-in-Time Compilation (JIT)">Chapter 32</a>).
Performing <acronym class="acronym">JIT</acronym> costs planning time but can
accelerate query execution.
Setting this to <code class="literal">-1</code> disables JIT compilation.
The default is <code class="literal">100000</code>.
</p></dd><dt id="GUC-JIT-INLINE-ABOVE-COST"><span class="term"><code class="varname">jit_inline_above_cost</code> (<code class="type">floating point</code>)
<a id="id-1.6.7.10.3.4.12.1.3" class="indexterm"></a>
</span></dt><dd><p>
Sets the query cost above which JIT compilation attempts to inline
functions and operators. Inlining adds planning time, but can
improve execution speed. It is not meaningful to set this to less
than <code class="varname">jit_above_cost</code>.
Setting this to <code class="literal">-1</code> disables inlining.
The default is <code class="literal">500000</code>.
</p></dd><dt id="GUC-JIT-OPTIMIZE-ABOVE-COST"><span class="term"><code class="varname">jit_optimize_above_cost</code> (<code class="type">floating point</code>)
<a id="id-1.6.7.10.3.4.13.1.3" class="indexterm"></a>
</span></dt><dd><p>
Sets the query cost above which JIT compilation applies expensive
optimizations. Such optimization adds planning time, but can improve
execution speed. It is not meaningful to set this to less
than <code class="varname">jit_above_cost</code>, and it is unlikely to be
beneficial to set it to more
than <code class="varname">jit_inline_above_cost</code>.
Setting this to <code class="literal">-1</code> disables expensive optimizations.
The default is <code class="literal">500000</code>.
</p></dd></dl></div></div><div class="sect2" id="RUNTIME-CONFIG-QUERY-GEQO"><div class="titlepage"><div><div><h3 class="title">20.7.3. Genetic Query Optimizer</h3></div></div></div><p>
The genetic query optimizer (GEQO) is an algorithm that does query
planning using heuristic searching. This reduces planning time for
complex queries (those joining many relations), at the cost of producing
plans that are sometimes inferior to those found by the normal
exhaustive-search algorithm.
For more information see <a class="xref" href="geqo.html" title="Chapter 62. Genetic Query Optimizer">Chapter 62</a>.
</p><div class="variablelist"><dl class="variablelist"><dt id="GUC-GEQO"><span class="term"><code class="varname">geqo</code> (<code class="type">boolean</code>)
<a id="id-1.6.7.10.4.3.1.1.3" class="indexterm"></a>
<a id="id-1.6.7.10.4.3.1.1.4" class="indexterm"></a>
<a id="id-1.6.7.10.4.3.1.1.5" class="indexterm"></a>
</span></dt><dd><p>
Enables or disables genetic query optimization.
This is on by default. It is usually best not to turn it off in
production; the <code class="varname">geqo_threshold</code> variable provides
more granular control of GEQO.
</p></dd><dt id="GUC-GEQO-THRESHOLD"><span class="term"><code class="varname">geqo_threshold</code> (<code class="type">integer</code>)
<a id="id-1.6.7.10.4.3.2.1.3" class="indexterm"></a>
</span></dt><dd><p>
Use genetic query optimization to plan queries with at least
this many <code class="literal">FROM</code> items involved. (Note that a
<code class="literal">FULL OUTER JOIN</code> construct counts as only one <code class="literal">FROM</code>
item.) The default is 12. For simpler queries it is usually best
to use the regular, exhaustive-search planner, but for queries with
many tables the exhaustive search takes too long, often
longer than the penalty of executing a suboptimal plan. Thus,
a threshold on the size of the query is a convenient way to manage
use of GEQO.
</p></dd><dt id="GUC-GEQO-EFFORT"><span class="term"><code class="varname">geqo_effort</code> (<code class="type">integer</code>)
<a id="id-1.6.7.10.4.3.3.1.3" class="indexterm"></a>
</span></dt><dd><p>
Controls the trade-off between planning time and query plan
quality in GEQO. This variable must be an integer in the
range from 1 to 10. The default value is five. Larger values
increase the time spent doing query planning, but also
increase the likelihood that an efficient query plan will be
chosen.
</p><p>
<code class="varname">geqo_effort</code> doesn't actually do anything
directly; it is only used to compute the default values for
the other variables that influence GEQO behavior (described
below). If you prefer, you can set the other parameters by
hand instead.
</p></dd><dt id="GUC-GEQO-POOL-SIZE"><span class="term"><code class="varname">geqo_pool_size</code> (<code class="type">integer</code>)
<a id="id-1.6.7.10.4.3.4.1.3" class="indexterm"></a>
</span></dt><dd><p>
Controls the pool size used by GEQO, that is the
number of individuals in the genetic population. It must be
at least two, and useful values are typically 100 to 1000. If
it is set to zero (the default setting) then a suitable
value is chosen based on <code class="varname">geqo_effort</code> and
the number of tables in the query.
</p></dd><dt id="GUC-GEQO-GENERATIONS"><span class="term"><code class="varname">geqo_generations</code> (<code class="type">integer</code>)
<a id="id-1.6.7.10.4.3.5.1.3" class="indexterm"></a>
</span></dt><dd><p>
Controls the number of generations used by GEQO, that is
the number of iterations of the algorithm. It must
be at least one, and useful values are in the same range as
the pool size. If it is set to zero (the default setting)
then a suitable value is chosen based on
<code class="varname">geqo_pool_size</code>.
</p></dd><dt id="GUC-GEQO-SELECTION-BIAS"><span class="term"><code class="varname">geqo_selection_bias</code> (<code class="type">floating point</code>)
<a id="id-1.6.7.10.4.3.6.1.3" class="indexterm"></a>
</span></dt><dd><p>
Controls the selection bias used by GEQO. The selection bias
is the selective pressure within the population. Values can be
from 1.50 to 2.00; the latter is the default.
</p></dd><dt id="GUC-GEQO-SEED"><span class="term"><code class="varname">geqo_seed</code> (<code class="type">floating point</code>)
<a id="id-1.6.7.10.4.3.7.1.3" class="indexterm"></a>
</span></dt><dd><p>
Controls the initial value of the random number generator used
by GEQO to select random paths through the join order search space.
The value can range from zero (the default) to one. Varying the
value changes the set of join paths explored, and may result in a
better or worse best path being found.
</p></dd></dl></div></div><div class="sect2" id="RUNTIME-CONFIG-QUERY-OTHER"><div class="titlepage"><div><div><h3 class="title">20.7.4. Other Planner Options</h3></div></div></div><div class="variablelist"><dl class="variablelist"><dt id="GUC-DEFAULT-STATISTICS-TARGET"><span class="term"><code class="varname">default_statistics_target</code> (<code class="type">integer</code>)
<a id="id-1.6.7.10.5.2.1.1.3" class="indexterm"></a>
</span></dt><dd><p>
Sets the default statistics target for table columns without
a column-specific target set via <code class="command">ALTER TABLE
SET STATISTICS</code>. Larger values increase the time needed to
do <code class="command">ANALYZE</code>, but might improve the quality of the
planner's estimates. The default is 100. For more information
on the use of statistics by the <span class="productname">PostgreSQL</span>
query planner, refer to <a class="xref" href="planner-stats.html" title="14.2. Statistics Used by the Planner">Section 14.2</a>.
</p></dd><dt id="GUC-CONSTRAINT-EXCLUSION"><span class="term"><code class="varname">constraint_exclusion</code> (<code class="type">enum</code>)
<a id="id-1.6.7.10.5.2.2.1.3" class="indexterm"></a>
<a id="id-1.6.7.10.5.2.2.1.4" class="indexterm"></a>
</span></dt><dd><p>
Controls the query planner's use of table constraints to
optimize queries.
The allowed values of <code class="varname">constraint_exclusion</code> are
<code class="literal">on</code> (examine constraints for all tables),
<code class="literal">off</code> (never examine constraints), and
<code class="literal">partition</code> (examine constraints only for inheritance
child tables and <code class="literal">UNION ALL</code> subqueries).
<code class="literal">partition</code> is the default setting.
It is often used with traditional inheritance trees to improve
performance.
</p><p>
When this parameter allows it for a particular table, the planner
compares query conditions with the table's <code class="literal">CHECK</code>
constraints, and omits scanning tables for which the conditions
contradict the constraints. For example:
</p><pre class="programlisting">
CREATE TABLE parent(key integer, ...);
CREATE TABLE child1000(check (key between 1000 and 1999)) INHERITS(parent);
CREATE TABLE child2000(check (key between 2000 and 2999)) INHERITS(parent);
...
SELECT * FROM parent WHERE key = 2400;
</pre><p>
With constraint exclusion enabled, this <code class="command">SELECT</code>
will not scan <code class="structname">child1000</code> at all, improving performance.
</p><p>
Currently, constraint exclusion is enabled by default
only for cases that are often used to implement table partitioning via
inheritance trees. Turning it on for all tables imposes extra
planning overhead that is quite noticeable on simple queries, and most
often will yield no benefit for simple queries. If you have no
tables that are partitioned using traditional inheritance, you might
prefer to turn it off entirely. (Note that the equivalent feature for
partitioned tables is controlled by a separate parameter,
<a class="xref" href="runtime-config-query.html#GUC-ENABLE-PARTITION-PRUNING">enable_partition_pruning</a>.)
</p><p>
Refer to <a class="xref" href="ddl-partitioning.html#DDL-PARTITIONING-CONSTRAINT-EXCLUSION" title="5.11.5. Partitioning and Constraint Exclusion">Section 5.11.5</a> for
more information on using constraint exclusion to implement
partitioning.
</p></dd><dt id="GUC-CURSOR-TUPLE-FRACTION"><span class="term"><code class="varname">cursor_tuple_fraction</code> (<code class="type">floating point</code>)
<a id="id-1.6.7.10.5.2.3.1.3" class="indexterm"></a>
</span></dt><dd><p>
Sets the planner's estimate of the fraction of a cursor's rows that
will be retrieved. The default is 0.1. Smaller values of this
setting bias the planner towards using <span class="quote">“<span class="quote">fast start</span>”</span> plans
for cursors, which will retrieve the first few rows quickly while
perhaps taking a long time to fetch all rows. Larger values
put more emphasis on the total estimated time. At the maximum
setting of 1.0, cursors are planned exactly like regular queries,
considering only the total estimated time and not how soon the
first rows might be delivered.
</p></dd><dt id="GUC-FROM-COLLAPSE-LIMIT"><span class="term"><code class="varname">from_collapse_limit</code> (<code class="type">integer</code>)
<a id="id-1.6.7.10.5.2.4.1.3" class="indexterm"></a>
</span></dt><dd><p>
The planner will merge sub-queries into upper queries if the
resulting <code class="literal">FROM</code> list would have no more than
this many items. Smaller values reduce planning time but might
yield inferior query plans. The default is eight.
For more information see <a class="xref" href="explicit-joins.html" title="14.3. Controlling the Planner with Explicit JOIN Clauses">Section 14.3</a>.
</p><p>
Setting this value to <a class="xref" href="runtime-config-query.html#GUC-GEQO-THRESHOLD">geqo_threshold</a> or more
may trigger use of the GEQO planner, resulting in non-optimal
plans. See <a class="xref" href="runtime-config-query.html#RUNTIME-CONFIG-QUERY-GEQO" title="20.7.3. Genetic Query Optimizer">Section 20.7.3</a>.
</p></dd><dt id="GUC-JIT"><span class="term"><code class="varname">jit</code> (<code class="type">boolean</code>)
<a id="id-1.6.7.10.5.2.5.1.3" class="indexterm"></a>
</span></dt><dd><p>
Determines whether <acronym class="acronym">JIT</acronym> compilation may be used by
<span class="productname">PostgreSQL</span>, if available (see <a class="xref" href="jit.html" title="Chapter 32. Just-in-Time Compilation (JIT)">Chapter 32</a>).
The default is <code class="literal">on</code>.
</p></dd><dt id="GUC-JOIN-COLLAPSE-LIMIT"><span class="term"><code class="varname">join_collapse_limit</code> (<code class="type">integer</code>)
<a id="id-1.6.7.10.5.2.6.1.3" class="indexterm"></a>
</span></dt><dd><p>
The planner will rewrite explicit <code class="literal">JOIN</code>
constructs (except <code class="literal">FULL JOIN</code>s) into lists of
<code class="literal">FROM</code> items whenever a list of no more than this many items
would result. Smaller values reduce planning time but might
yield inferior query plans.
</p><p>
By default, this variable is set the same as
<code class="varname">from_collapse_limit</code>, which is appropriate
for most uses. Setting it to 1 prevents any reordering of
explicit <code class="literal">JOIN</code>s. Thus, the explicit join order
specified in the query will be the actual order in which the
relations are joined. Because the query planner does not always choose
the optimal join order, advanced users can elect to
temporarily set this variable to 1, and then specify the join
order they desire explicitly.
For more information see <a class="xref" href="explicit-joins.html" title="14.3. Controlling the Planner with Explicit JOIN Clauses">Section 14.3</a>.
</p><p>
Setting this value to <a class="xref" href="runtime-config-query.html#GUC-GEQO-THRESHOLD">geqo_threshold</a> or more
may trigger use of the GEQO planner, resulting in non-optimal
plans. See <a class="xref" href="runtime-config-query.html#RUNTIME-CONFIG-QUERY-GEQO" title="20.7.3. Genetic Query Optimizer">Section 20.7.3</a>.
</p></dd><dt id="GUC-PLAN-CACHE_MODE"><span class="term"><code class="varname">plan_cache_mode</code> (<code class="type">enum</code>)
<a id="id-1.6.7.10.5.2.7.1.3" class="indexterm"></a>
</span></dt><dd><p>
Prepared statements (either explicitly prepared or implicitly
generated, for example by PL/pgSQL) can be executed using custom or
generic plans. Custom plans are made afresh for each execution
using its specific set of parameter values, while generic plans do
not rely on the parameter values and can be re-used across
executions. Thus, use of a generic plan saves planning time, but if
the ideal plan depends strongly on the parameter values then a
generic plan may be inefficient. The choice between these options
is normally made automatically, but it can be overridden
with <code class="varname">plan_cache_mode</code>.
The allowed values are <code class="literal">auto</code> (the default),
<code class="literal">force_custom_plan</code> and
<code class="literal">force_generic_plan</code>.
This setting is considered when a cached plan is to be executed,
not when it is prepared.
For more information see <a class="xref" href="sql-prepare.html" title="PREPARE"><span class="refentrytitle">PREPARE</span></a>.
</p></dd><dt id="GUC-RECURSIVE-WORKTABLE-FACTOR"><span class="term"><code class="varname">recursive_worktable_factor</code> (<code class="type">floating point</code>)
<a id="id-1.6.7.10.5.2.8.1.3" class="indexterm"></a>
</span></dt><dd><p>
Sets the planner's estimate of the average size of the working
table of a <a class="link" href="queries-with.html#QUERIES-WITH-RECURSIVE" title="7.8.2. Recursive Queries">recursive
query</a>, as a multiple of the estimated size of the initial
non-recursive term of the query. This helps the planner choose
the most appropriate method for joining the working table to the
query's other tables.
The default value is <code class="literal">10.0</code>. A smaller value
such as <code class="literal">1.0</code> can be helpful when the recursion
has low <span class="quote">“<span class="quote">fan-out</span>”</span> from one step to the next, as for
example in shortest-path queries. Graph analytics queries may
benefit from larger-than-default values.
</p></dd></dl></div></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="runtime-config-replication.html" title="20.6. Replication">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="runtime-config.html" title="Chapter 20. Server Configuration">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="runtime-config-logging.html" title="20.8. Error Reporting and Logging">Next</a></td></tr><tr><td width="40%" align="left" valign="top">20.6. Replication </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.7 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 20.8. Error Reporting and Logging</td></tr></table></div></body></html>
|
