1EXPLAIN(7)               PostgreSQL 13.3 Documentation              EXPLAIN(7)
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NAME

6       EXPLAIN - show the execution plan of a statement
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SYNOPSIS

9       EXPLAIN [ ( option [, ...] ) ] statement
10       EXPLAIN [ ANALYZE ] [ VERBOSE ] statement
11
12       where option can be one of:
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14           ANALYZE [ boolean ]
15           VERBOSE [ boolean ]
16           COSTS [ boolean ]
17           SETTINGS [ boolean ]
18           BUFFERS [ boolean ]
19           WAL [ boolean ]
20           TIMING [ boolean ]
21           SUMMARY [ boolean ]
22           FORMAT { TEXT | XML | JSON | YAML }
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DESCRIPTION

25       This command displays the execution plan that the PostgreSQL planner
26       generates for the supplied statement. The execution plan shows how the
27       table(s) referenced by the statement will be scanned — by plain
28       sequential scan, index scan, etc. — and if multiple tables are
29       referenced, what join algorithms will be used to bring together the
30       required rows from each input table.
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32       The most critical part of the display is the estimated statement
33       execution cost, which is the planner's guess at how long it will take
34       to run the statement (measured in cost units that are arbitrary, but
35       conventionally mean disk page fetches). Actually two numbers are shown:
36       the start-up cost before the first row can be returned, and the total
37       cost to return all the rows. For most queries the total cost is what
38       matters, but in contexts such as a subquery in EXISTS, the planner will
39       choose the smallest start-up cost instead of the smallest total cost
40       (since the executor will stop after getting one row, anyway). Also, if
41       you limit the number of rows to return with a LIMIT clause, the planner
42       makes an appropriate interpolation between the endpoint costs to
43       estimate which plan is really the cheapest.
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45       The ANALYZE option causes the statement to be actually executed, not
46       only planned. Then actual run time statistics are added to the display,
47       including the total elapsed time expended within each plan node (in
48       milliseconds) and the total number of rows it actually returned. This
49       is useful for seeing whether the planner's estimates are close to
50       reality.
51
52           Important
53           Keep in mind that the statement is actually executed when the
54           ANALYZE option is used. Although EXPLAIN will discard any output
55           that a SELECT would return, other side effects of the statement
56           will happen as usual. If you wish to use EXPLAIN ANALYZE on an
57           INSERT, UPDATE, DELETE, CREATE TABLE AS, or EXECUTE statement
58           without letting the command affect your data, use this approach:
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60               BEGIN;
61               EXPLAIN ANALYZE ...;
62               ROLLBACK;
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64       Only the ANALYZE and VERBOSE options can be specified, and only in that
65       order, without surrounding the option list in parentheses. Prior to
66       PostgreSQL 9.0, the unparenthesized syntax was the only one supported.
67       It is expected that all new options will be supported only in the
68       parenthesized syntax.
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PARAMETERS

71       ANALYZE
72           Carry out the command and show actual run times and other
73           statistics. This parameter defaults to FALSE.
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75       VERBOSE
76           Display additional information regarding the plan. Specifically,
77           include the output column list for each node in the plan tree,
78           schema-qualify table and function names, always label variables in
79           expressions with their range table alias, and always print the name
80           of each trigger for which statistics are displayed. This parameter
81           defaults to FALSE.
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83       COSTS
84           Include information on the estimated startup and total cost of each
85           plan node, as well as the estimated number of rows and the
86           estimated width of each row. This parameter defaults to TRUE.
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88       SETTINGS
89           Include information on configuration parameters. Specifically,
90           include options affecting query planning with value different from
91           the built-in default value. This parameter defaults to FALSE.
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93       BUFFERS
94           Include information on buffer usage. Specifically, include the
95           number of shared blocks hit, read, dirtied, and written, the number
96           of local blocks hit, read, dirtied, and written, the number of temp
97           blocks read and written, and the time spent reading and writing
98           data file blocks (in milliseconds) if track_io_timing is enabled. A
99           hit means that a read was avoided because the block was found
100           already in cache when needed. Shared blocks contain data from
101           regular tables and indexes; local blocks contain data from
102           temporary tables and indexes; while temp blocks contain short-term
103           working data used in sorts, hashes, Materialize plan nodes, and
104           similar cases. The number of blocks dirtied indicates the number of
105           previously unmodified blocks that were changed by this query; while
106           the number of blocks written indicates the number of
107           previously-dirtied blocks evicted from cache by this backend during
108           query processing. The number of blocks shown for an upper-level
109           node includes those used by all its child nodes. In text format,
110           only non-zero values are printed. It defaults to FALSE.
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112       WAL
113           Include information on WAL record generation. Specifically, include
114           the number of records, number of full page images (fpi) and the
115           amount of WAL generated in bytes. In text format, only non-zero
116           values are printed. This parameter may only be used when ANALYZE is
117           also enabled. It defaults to FALSE.
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119       TIMING
120           Include actual startup time and time spent in each node in the
121           output. The overhead of repeatedly reading the system clock can
122           slow down the query significantly on some systems, so it may be
123           useful to set this parameter to FALSE when only actual row counts,
124           and not exact times, are needed. Run time of the entire statement
125           is always measured, even when node-level timing is turned off with
126           this option. This parameter may only be used when ANALYZE is also
127           enabled. It defaults to TRUE.
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129       SUMMARY
130           Include summary information (e.g., totaled timing information)
131           after the query plan. Summary information is included by default
132           when ANALYZE is used but otherwise is not included by default, but
133           can be enabled using this option. Planning time in EXPLAIN EXECUTE
134           includes the time required to fetch the plan from the cache and the
135           time required for re-planning, if necessary.
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137       FORMAT
138           Specify the output format, which can be TEXT, XML, JSON, or YAML.
139           Non-text output contains the same information as the text output
140           format, but is easier for programs to parse. This parameter
141           defaults to TEXT.
142
143       boolean
144           Specifies whether the selected option should be turned on or off.
145           You can write TRUE, ON, or 1 to enable the option, and FALSE, OFF,
146           or 0 to disable it. The boolean value can also be omitted, in which
147           case TRUE is assumed.
148
149       statement
150           Any SELECT, INSERT, UPDATE, DELETE, VALUES, EXECUTE, DECLARE,
151           CREATE TABLE AS, or CREATE MATERIALIZED VIEW AS statement, whose
152           execution plan you wish to see.
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OUTPUTS

155       The command's result is a textual description of the plan selected for
156       the statement, optionally annotated with execution statistics.
157       Section 14.1 describes the information provided.
158

NOTES

160       In order to allow the PostgreSQL query planner to make reasonably
161       informed decisions when optimizing queries, the pg_statistic data
162       should be up-to-date for all tables used in the query. Normally the
163       autovacuum daemon will take care of that automatically. But if a table
164       has recently had substantial changes in its contents, you might need to
165       do a manual ANALYZE(7) rather than wait for autovacuum to catch up with
166       the changes.
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168       In order to measure the run-time cost of each node in the execution
169       plan, the current implementation of EXPLAIN ANALYZE adds profiling
170       overhead to query execution. As a result, running EXPLAIN ANALYZE on a
171       query can sometimes take significantly longer than executing the query
172       normally. The amount of overhead depends on the nature of the query, as
173       well as the platform being used. The worst case occurs for plan nodes
174       that in themselves require very little time per execution, and on
175       machines that have relatively slow operating system calls for obtaining
176       the time of day.
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EXAMPLES

179       To show the plan for a simple query on a table with a single integer
180       column and 10000 rows:
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182           EXPLAIN SELECT * FROM foo;
183
184                                  QUERY PLAN
185           ---------------------------------------------------------
186            Seq Scan on foo  (cost=0.00..155.00 rows=10000 width=4)
187           (1 row)
188
189       Here is the same query, with JSON output formatting:
190
191           EXPLAIN (FORMAT JSON) SELECT * FROM foo;
192                      QUERY PLAN
193           --------------------------------
194            [                             +
195              {                           +
196                "Plan": {                 +
197                  "Node Type": "Seq Scan",+
198                  "Relation Name": "foo", +
199                  "Alias": "foo",         +
200                  "Startup Cost": 0.00,   +
201                  "Total Cost": 155.00,   +
202                  "Plan Rows": 10000,     +
203                  "Plan Width": 4         +
204                }                         +
205              }                           +
206            ]
207           (1 row)
208
209       If there is an index and we use a query with an indexable WHERE
210       condition, EXPLAIN might show a different plan:
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212           EXPLAIN SELECT * FROM foo WHERE i = 4;
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214                                    QUERY PLAN
215           --------------------------------------------------------------
216            Index Scan using fi on foo  (cost=0.00..5.98 rows=1 width=4)
217              Index Cond: (i = 4)
218           (2 rows)
219
220       Here is the same query, but in YAML format:
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222           EXPLAIN (FORMAT YAML) SELECT * FROM foo WHERE i='4';
223                     QUERY PLAN
224           -------------------------------
225            - Plan:                      +
226                Node Type: "Index Scan"  +
227                Scan Direction: "Forward"+
228                Index Name: "fi"         +
229                Relation Name: "foo"     +
230                Alias: "foo"             +
231                Startup Cost: 0.00       +
232                Total Cost: 5.98         +
233                Plan Rows: 1             +
234                Plan Width: 4            +
235                Index Cond: "(i = 4)"
236           (1 row)
237
238       XML format is left as an exercise for the reader.
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240       Here is the same plan with cost estimates suppressed:
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242           EXPLAIN (COSTS FALSE) SELECT * FROM foo WHERE i = 4;
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244                   QUERY PLAN
245           ----------------------------
246            Index Scan using fi on foo
247              Index Cond: (i = 4)
248           (2 rows)
249
250       Here is an example of a query plan for a query using an aggregate
251       function:
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253           EXPLAIN SELECT sum(i) FROM foo WHERE i < 10;
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255                                        QUERY PLAN
256           ---------------------------------------------------------------------
257            Aggregate  (cost=23.93..23.93 rows=1 width=4)
258              ->  Index Scan using fi on foo  (cost=0.00..23.92 rows=6 width=4)
259                    Index Cond: (i < 10)
260           (3 rows)
261
262       Here is an example of using EXPLAIN EXECUTE to display the execution
263       plan for a prepared query:
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265           PREPARE query(int, int) AS SELECT sum(bar) FROM test
266               WHERE id > $1 AND id < $2
267               GROUP BY foo;
268
269           EXPLAIN ANALYZE EXECUTE query(100, 200);
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271                                                                  QUERY PLAN
272           ------------------------------------------------------------------------------------------------------------------------
273            HashAggregate  (cost=9.54..9.54 rows=1 width=8) (actual time=0.156..0.161 rows=11 loops=1)
274              Group Key: foo
275              ->  Index Scan using test_pkey on test  (cost=0.29..9.29 rows=50 width=8) (actual time=0.039..0.091 rows=99 loops=1)
276                    Index Cond: ((id > $1) AND (id < $2))
277            Planning time: 0.197 ms
278            Execution time: 0.225 ms
279           (6 rows)
280
281       Of course, the specific numbers shown here depend on the actual
282       contents of the tables involved. Also note that the numbers, and even
283       the selected query strategy, might vary between PostgreSQL releases due
284       to planner improvements. In addition, the ANALYZE command uses random
285       sampling to estimate data statistics; therefore, it is possible for
286       cost estimates to change after a fresh run of ANALYZE, even if the
287       actual distribution of data in the table has not changed.
288

COMPATIBILITY

290       There is no EXPLAIN statement defined in the SQL standard.
291

SEE ALSO

293       ANALYZE(7)
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295
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297PostgreSQL 13.3                      2021                           EXPLAIN(7)
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