From 3c05284d83b230728e59a25e828992037ef77096 Mon Sep 17 00:00:00 2001
From: Tom Lane
Date: Fri, 24 Mar 2023 17:07:14 -0400
Subject: Invent GENERIC_PLAN option for EXPLAIN.
This provides a very simple way to see the generic plan for a
parameterized query. Without this, it's necessary to define
a prepared statement and temporarily change plan_cache_mode,
which is a bit tedious.
One thing that's a bit of a hack perhaps is that we disable
execution-time partition pruning when the GENERIC_PLAN option
is given. That's because the pruning code may attempt to
fetch the value of one of the parameters, which would fail.
Laurenz Albe, reviewed by Julien Rouhaud, Christoph Berg,
Michel Pelletier, Jim Jones, and myself
Discussion: https://postgr.es/m/0a29b954b10b57f0d135fe12aa0909bd41883eb0.camel@cybertec.at
---
doc/src/sgml/ref/explain.sgml | 70 ++++++++++++++++++++++++++++++++++++++-----
1 file changed, 62 insertions(+), 8 deletions(-)
(limited to 'doc/src')
diff --git a/doc/src/sgml/ref/explain.sgml b/doc/src/sgml/ref/explain.sgml
index 0fce6224232..410490951b1 100644
--- a/doc/src/sgml/ref/explain.sgml
+++ b/doc/src/sgml/ref/explain.sgml
@@ -40,6 +40,7 @@ EXPLAIN [ ANALYZE ] [ VERBOSE ] statementboolean ]
COSTS [ boolean ]
SETTINGS [ boolean ]
+ GENERIC_PLAN [ boolean ]
BUFFERS [ boolean ]
WAL [ boolean ]
TIMING [ boolean ]
@@ -168,6 +169,22 @@ ROLLBACK;
+
+ GENERIC_PLAN
+
+
+ Allow the statement to contain parameter placeholders like
+ $1, and generate a generic plan that does not
+ depend on the values of those parameters.
+ See PREPARE
+ for details about generic plans and the types of statement that
+ support parameters.
+ This parameter cannot be used together with ANALYZE.
+ It defaults to FALSE.
+
+
+
+
BUFFERS
@@ -191,7 +208,7 @@ ROLLBACK;
query processing.
The number of blocks shown for an
upper-level node includes those used by all its child nodes. In text
- format, only non-zero values are printed. It defaults to
+ format, only non-zero values are printed. This parameter defaults to
FALSE.
@@ -445,14 +462,15 @@ PREPARE query(int, int) AS SELECT sum(bar) FROM test
EXPLAIN ANALYZE EXECUTE query(100, 200);
QUERY PLAN
--------------------------------------------------------------------&zwsp;-----------------------------------------------------
- HashAggregate (cost=9.54..9.54 rows=1 width=8) (actual time=0.156..0.161 rows=11 loops=1)
+-------------------------------------------------------------------&zwsp;------------------------------------------------------
+ HashAggregate (cost=10.77..10.87 rows=10 width=12) (actual time=0.043..0.044 rows=10 loops=1)
Group Key: foo
- -> 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)
- Index Cond: ((id > $1) AND (id < $2))
- Planning time: 0.197 ms
- Execution time: 0.225 ms
-(6 rows)
+ Batches: 1 Memory Usage: 24kB
+ -> Index Scan using test_pkey on test (cost=0.29..10.27 rows=99 width=8) (actual time=0.009..0.025 rows=99 loops=1)
+ Index Cond: ((id > 100) AND (id < 200))
+ Planning Time: 0.244 ms
+ Execution Time: 0.073 ms
+(7 rows)
@@ -467,6 +485,42 @@ EXPLAIN ANALYZE EXECUTE query(100, 200);
ANALYZE, even if the actual distribution of data
in the table has not changed.
+
+
+ Notice that the previous example showed a custom plan
+ for the specific parameter values given in EXECUTE.
+ We might also wish to see the generic plan for a parameterized
+ query, which can be done with GENERIC_PLAN:
+
+
+EXPLAIN (GENERIC_PLAN)
+ SELECT sum(bar) FROM test
+ WHERE id > $1 AND id < $2
+ GROUP BY foo;
+
+ QUERY PLAN
+-------------------------------------------------------------------&zwsp;------------
+ HashAggregate (cost=26.79..26.89 rows=10 width=12)
+ Group Key: foo
+ -> Index Scan using test_pkey on test (cost=0.29..24.29 rows=500 width=8)
+ Index Cond: ((id > $1) AND (id < $2))
+(4 rows)
+
+
+ In this case the parser correctly inferred that $1
+ and $2 should have the same data type
+ as id, so the lack of parameter type information
+ from PREPARE was not a problem. In other cases
+ it might be necessary to explicitly specify types for the parameter
+ symbols, which can be done by casting them, for example:
+
+
+EXPLAIN (GENERIC_PLAN)
+ SELECT sum(bar) FROM test
+ WHERE id > $1::integer AND id < $2::integer
+ GROUP BY foo;
+
+
--
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