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Invent GENERIC_PLAN option for EXPLAIN.

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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
This commit is contained in:
Tom Lane 2023-03-24 17:07:14 -04:00
parent 5b140dc8f0
commit 3c05284d83
10 changed files with 197 additions and 20 deletions
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70
doc/src/sgml/ref/explain.sgml
View File

@ -40,6 +40,7 @@ EXPLAIN [ ANALYZE ] [ VERBOSE ] <replaceable class="parameter">statement</replac
VERBOSE [ <replaceable class="parameter">boolean</replaceable> ]
COSTS [ <replaceable class="parameter">boolean</replaceable> ]
SETTINGS [ <replaceable class="parameter">boolean</replaceable> ]
GENERIC_PLAN [ <replaceable class="parameter">boolean</replaceable> ]
BUFFERS [ <replaceable class="parameter">boolean</replaceable> ]
WAL [ <replaceable class="parameter">boolean</replaceable> ]
TIMING [ <replaceable class="parameter">boolean</replaceable> ]
@ -168,6 +169,22 @@ ROLLBACK;
</listitem>
</varlistentry>
<varlistentry>
<term><literal>GENERIC_PLAN</literal></term>
<listitem>
<para>
Allow the statement to contain parameter placeholders like
<literal>$1</literal>, and generate a generic plan that does not
depend on the values of those parameters.
See <link linkend="sql-prepare"><command>PREPARE</command></link>
for details about generic plans and the types of statement that
support parameters.
This parameter cannot be used together with <literal>ANALYZE</literal>.
It defaults to <literal>FALSE</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>BUFFERS</literal></term>
<listitem>
@ -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
<literal>FALSE</literal>.
</para>
</listitem>
@ -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
-&gt; 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 &gt; $1) AND (id &lt; $2))
Planning time: 0.197 ms
Execution time: 0.225 ms
(6 rows)
Batches: 1 Memory Usage: 24kB
-&gt; 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 &gt; 100) AND (id &lt; 200))
Planning Time: 0.244 ms
Execution Time: 0.073 ms
(7 rows)
</programlisting>
</para>
@ -467,6 +485,42 @@ EXPLAIN ANALYZE EXECUTE query(100, 200);
<command>ANALYZE</command>, even if the actual distribution of data
in the table has not changed.
</para>
<para>
Notice that the previous example showed a <quote>custom</quote> plan
for the specific parameter values given in <command>EXECUTE</command>.
We might also wish to see the generic plan for a parameterized
query, which can be done with <literal>GENERIC_PLAN</literal>:
<programlisting>
EXPLAIN (GENERIC_PLAN)
SELECT sum(bar) FROM test
WHERE id &gt; $1 AND id &lt; $2
GROUP BY foo;
QUERY PLAN
-------------------------------------------------------------------&zwsp;------------
HashAggregate (cost=26.79..26.89 rows=10 width=12)
Group Key: foo
-&gt; Index Scan using test_pkey on test (cost=0.29..24.29 rows=500 width=8)
Index Cond: ((id &gt; $1) AND (id &lt; $2))
(4 rows)
</programlisting>
In this case the parser correctly inferred that <literal>$1</literal>
and <literal>$2</literal> should have the same data type
as <literal>id</literal>, so the lack of parameter type information
from <command>PREPARE</command> 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:
<programlisting>
EXPLAIN (GENERIC_PLAN)
SELECT sum(bar) FROM test
WHERE id &gt; $1::integer AND id &lt; $2::integer
GROUP BY foo;
</programlisting>
</para>
</refsect1>
<refsect1>

11
src/backend/commands/explain.c
View File

@ -190,6 +190,8 @@ ExplainQuery(ParseState *pstate, ExplainStmt *stmt,
es->wal = defGetBoolean(opt);
else if (strcmp(opt->defname, "settings") == 0)
es->settings = defGetBoolean(opt);
else if (strcmp(opt->defname, "generic_plan") == 0)
es->generic = defGetBoolean(opt);
else if (strcmp(opt->defname, "timing") == 0)
{
timing_set = true;
@ -227,6 +229,7 @@ ExplainQuery(ParseState *pstate, ExplainStmt *stmt,
parser_errposition(pstate, opt->location)));
}
/* check that WAL is used with EXPLAIN ANALYZE */
if (es->wal && !es->analyze)
ereport(ERROR,
(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
@ -241,6 +244,12 @@ ExplainQuery(ParseState *pstate, ExplainStmt *stmt,
(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
errmsg("EXPLAIN option TIMING requires ANALYZE")));
/* check that GENERIC_PLAN is not used with EXPLAIN ANALYZE */
if (es->generic && es->analyze)
ereport(ERROR,
(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
errmsg("EXPLAIN options ANALYZE and GENERIC_PLAN cannot be used together")));
/* if the summary was not set explicitly, set default value */
es->summary = (summary_set) ? es->summary : es->analyze;
@ -572,6 +581,8 @@ ExplainOnePlan(PlannedStmt *plannedstmt, IntoClause *into, ExplainState *es,
eflags = 0; /* default run-to-completion flags */
else
eflags = EXEC_FLAG_EXPLAIN_ONLY;
if (es->generic)
eflags |= EXEC_FLAG_EXPLAIN_GENERIC;
if (into)
eflags |= GetIntoRelEFlags(into);

2
src/backend/executor/execMain.c
View File

@ -911,7 +911,7 @@ InitPlan(QueryDesc *queryDesc, int eflags)
* prepared to handle REWIND efficiently; otherwise there is no need.
*/
sp_eflags = eflags
& (EXEC_FLAG_EXPLAIN_ONLY | EXEC_FLAG_WITH_NO_DATA);
& ~(EXEC_FLAG_REWIND | EXEC_FLAG_BACKWARD | EXEC_FLAG_MARK);
if (bms_is_member(i, plannedstmt->rewindPlanIDs))
sp_eflags |= EXEC_FLAG_REWIND;

10
src/backend/executor/execPartition.c
View File

@ -2044,10 +2044,13 @@ CreatePartitionPruneState(PlanState *planstate, PartitionPruneInfo *pruneinfo)
pprune->present_parts = bms_copy(pinfo->present_parts);
/*
* Initialize pruning contexts as needed.
* Initialize pruning contexts as needed. Note that we must skip
* execution-time partition pruning in EXPLAIN (GENERIC_PLAN),
* since parameter values may be missing.
*/
pprune->initial_pruning_steps = pinfo->initial_pruning_steps;
if (pinfo->initial_pruning_steps)
if (pinfo->initial_pruning_steps &&
!(econtext->ecxt_estate->es_top_eflags & EXEC_FLAG_EXPLAIN_GENERIC))
{
InitPartitionPruneContext(&pprune->initial_context,
pinfo->initial_pruning_steps,
@ -2057,7 +2060,8 @@ CreatePartitionPruneState(PlanState *planstate, PartitionPruneInfo *pruneinfo)
prunestate->do_initial_prune = true;
}
pprune->exec_pruning_steps = pinfo->exec_pruning_steps;
if (pinfo->exec_pruning_steps)
if (pinfo->exec_pruning_steps &&
!(econtext->ecxt_estate->es_top_eflags & EXEC_FLAG_EXPLAIN_GENERIC))
{
InitPartitionPruneContext(&pprune->exec_context,
pinfo->exec_pruning_steps,

29
src/backend/parser/analyze.c
View File

@ -27,6 +27,7 @@
#include "access/sysattr.h"
#include "catalog/pg_proc.h"
#include "catalog/pg_type.h"
#include "commands/defrem.h"
#include "miscadmin.h"
#include "nodes/makefuncs.h"
#include "nodes/nodeFuncs.h"
@ -2906,10 +2907,38 @@ static Query *
transformExplainStmt(ParseState *pstate, ExplainStmt *stmt)
{
Query *result;
bool generic_plan = false;
Oid *paramTypes = NULL;
int numParams = 0;
/*
* If we have no external source of parameter definitions, and the
* GENERIC_PLAN option is specified, then accept variable parameter
* definitions (similarly to PREPARE, for example).
*/
if (pstate->p_paramref_hook == NULL)
{
ListCell *lc;
foreach(lc, stmt->options)
{
DefElem *opt = (DefElem *) lfirst(lc);
if (strcmp(opt->defname, "generic_plan") == 0)
generic_plan = defGetBoolean(opt);
/* don't "break", as we want the last value */
}
if (generic_plan)
setup_parse_variable_parameters(pstate, &paramTypes, &numParams);
}
/* transform contained query, allowing SELECT INTO */
stmt->query = (Node *) transformOptionalSelectInto(pstate, stmt->query);
/* make sure all is well with parameter types */
if (generic_plan)
check_variable_parameters(pstate, (Query *) stmt->query);
/* represent the command as a utility Query */
result = makeNode(Query);
result->commandType = CMD_UTILITY;

4
src/bin/psql/tab-complete.c
View File

@ -3752,9 +3752,9 @@ psql_completion(const char *text, int start, int end)
* one word, so the above test is correct.
*/
if (ends_with(prev_wd, '(') || ends_with(prev_wd, ','))
COMPLETE_WITH("ANALYZE", "VERBOSE", "COSTS", "SETTINGS",
COMPLETE_WITH("ANALYZE", "VERBOSE", "COSTS", "SETTINGS", "GENERIC_PLAN",
"BUFFERS", "WAL", "TIMING", "SUMMARY", "FORMAT");
else if (TailMatches("ANALYZE|VERBOSE|COSTS|SETTINGS|BUFFERS|WAL|TIMING|SUMMARY"))
else if (TailMatches("ANALYZE|VERBOSE|COSTS|SETTINGS|GENERIC_PLAN|BUFFERS|WAL|TIMING|SUMMARY"))
COMPLETE_WITH("ON", "OFF");
else if (TailMatches("FORMAT"))
COMPLETE_WITH("TEXT", "XML", "JSON", "YAML");

1
src/include/commands/explain.h
View File

@ -46,6 +46,7 @@ typedef struct ExplainState
bool timing; /* print detailed node timing */
bool summary; /* print total planning and execution timing */
bool settings; /* print modified settings */
bool generic; /* generate a generic plan */
ExplainFormat format; /* output format */
/* state for output formatting --- not reset for each new plan tree */
int indent; /* current indentation level */

22
src/include/executor/executor.h
View File

@ -36,6 +36,11 @@
* of startup should occur. However, error checks (such as permission checks)
* should be performed.
*
* EXPLAIN_GENERIC can only be used together with EXPLAIN_ONLY. It indicates
* that a generic plan is being shown using EXPLAIN (GENERIC_PLAN), which
* means that missing parameter values must be tolerated. Currently, the only
* effect is to suppress execution-time partition pruning.
*
* REWIND indicates that the plan node should try to efficiently support
* rescans without parameter changes. (Nodes must support ExecReScan calls
* in any case, but if this flag was not given, they are at liberty to do it
@ -52,13 +57,18 @@
* AfterTriggerBeginQuery/AfterTriggerEndQuery. This does not necessarily
* mean that the plan can't queue any AFTER triggers; just that the caller
* is responsible for there being a trigger context for them to be queued in.
*
* WITH_NO_DATA indicates that we are performing REFRESH MATERIALIZED VIEW
* ... WITH NO DATA. Currently, the only effect is to suppress errors about
* scanning unpopulated materialized views.
*/
#define EXEC_FLAG_EXPLAIN_ONLY 0x0001 /* EXPLAIN, no ANALYZE */
#define EXEC_FLAG_REWIND 0x0002 /* need efficient rescan */
#define EXEC_FLAG_BACKWARD 0x0004 /* need backward scan */
#define EXEC_FLAG_MARK 0x0008 /* need mark/restore */
#define EXEC_FLAG_SKIP_TRIGGERS 0x0010 /* skip AfterTrigger calls */
#define EXEC_FLAG_WITH_NO_DATA 0x0020 /* rel scannability doesn't matter */
#define EXEC_FLAG_EXPLAIN_ONLY 0x0001 /* EXPLAIN, no ANALYZE */
#define EXEC_FLAG_EXPLAIN_GENERIC 0x0002 /* EXPLAIN (GENERIC_PLAN) */
#define EXEC_FLAG_REWIND 0x0004 /* need efficient rescan */
#define EXEC_FLAG_BACKWARD 0x0008 /* need backward scan */
#define EXEC_FLAG_MARK 0x0010 /* need mark/restore */
#define EXEC_FLAG_SKIP_TRIGGERS 0x0020 /* skip AfterTrigger setup */
#define EXEC_FLAG_WITH_NO_DATA 0x0040 /* REFRESH ... WITH NO DATA */
/* Hook for plugins to get control in ExecutorStart() */

42
src/test/regress/expected/explain.out
View File

@ -308,6 +308,48 @@ select explain_filter_to_json('explain (settings, format json) select * from int
(1 row)
rollback;
-- GENERIC_PLAN option
select explain_filter('explain (generic_plan) select unique1 from tenk1 where thousand = $1');
explain_filter
---------------------------------------------------------------------------------
Bitmap Heap Scan on tenk1 (cost=N.N..N.N rows=N width=N)
Recheck Cond: (thousand = $N)
-> Bitmap Index Scan on tenk1_thous_tenthous (cost=N.N..N.N rows=N width=N)
Index Cond: (thousand = $N)
(4 rows)
-- should fail
select explain_filter('explain (analyze, generic_plan) select unique1 from tenk1 where thousand = $1');
ERROR: EXPLAIN options ANALYZE and GENERIC_PLAN cannot be used together
CONTEXT: PL/pgSQL function explain_filter(text) line 5 at FOR over EXECUTE statement
-- Test EXPLAIN (GENERIC_PLAN) with partition pruning
-- partitions should be pruned at plan time, based on constants,
-- but there should be no pruning based on parameter placeholders
create table gen_part (
key1 integer not null,
key2 integer not null
) partition by list (key1);
create table gen_part_1
partition of gen_part for values in (1)
partition by range (key2);
create table gen_part_1_1
partition of gen_part_1 for values from (1) to (2);
create table gen_part_1_2
partition of gen_part_1 for values from (2) to (3);
create table gen_part_2
partition of gen_part for values in (2);
-- should scan gen_part_1_1 and gen_part_1_2, but not gen_part_2
select explain_filter('explain (generic_plan) select key1, key2 from gen_part where key1 = 1 and key2 = $1');
explain_filter
---------------------------------------------------------------------------
Append (cost=N.N..N.N rows=N width=N)
-> Seq Scan on gen_part_1_1 gen_part_1 (cost=N.N..N.N rows=N width=N)
Filter: ((key1 = N) AND (key2 = $N))
-> Seq Scan on gen_part_1_2 gen_part_2 (cost=N.N..N.N rows=N width=N)
Filter: ((key1 = N) AND (key2 = $N))
(5 rows)
drop table gen_part;
--
-- Test production of per-worker data
--

26
src/test/regress/sql/explain.sql
View File

@ -88,6 +88,32 @@ select true as "OK"
select explain_filter_to_json('explain (settings, format json) select * from int8_tbl i8') #> '{0,Settings,plan_cache_mode}';
rollback;
-- GENERIC_PLAN option
select explain_filter('explain (generic_plan) select unique1 from tenk1 where thousand = $1');
-- should fail
select explain_filter('explain (analyze, generic_plan) select unique1 from tenk1 where thousand = $1');
-- Test EXPLAIN (GENERIC_PLAN) with partition pruning
-- partitions should be pruned at plan time, based on constants,
-- but there should be no pruning based on parameter placeholders
create table gen_part (
key1 integer not null,
key2 integer not null
) partition by list (key1);
create table gen_part_1
partition of gen_part for values in (1)
partition by range (key2);
create table gen_part_1_1
partition of gen_part_1 for values from (1) to (2);
create table gen_part_1_2
partition of gen_part_1 for values from (2) to (3);
create table gen_part_2
partition of gen_part for values in (2);
-- should scan gen_part_1_1 and gen_part_1_2, but not gen_part_2
select explain_filter('explain (generic_plan) select key1, key2 from gen_part where key1 = 1 and key2 = $1');
drop table gen_part;
--
-- Test production of per-worker data
--