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Commitbf61873

Browse files
author
Amit Kapila
committed
Prohibit pushing subqueries containing window function calculation to
workers.Allowing window function calculation in workers leads to inconsistentresults because if the input row ordering is not fully deterministic, theoutput of window functions might vary across workers. The fix is to treatthem as parallel-restricted.In the passing, improve the coding pattern in max_parallel_hazard_walkerso that it has a chain of mutually-exclusive if ... else if ... else if... else if ... IsA tests.Reported-by: Marko TiikkajaBug: 15324Author: Amit KapilaReviewed-by: Tom LaneBackpatch-through: 9.6Discussion:https://postgr.es/m/CAL9smLAnfPJCDUUG4ckX2iznj53V7VSMsYefzZieN93YxTNOcw@mail.gmail.com
1 parent3b7a96a commitbf61873

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3 files changed

+47
-1
lines changed

3 files changed

+47
-1
lines changed

‎src/backend/optimizer/util/clauses.c

Lines changed: 15 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -1168,12 +1168,26 @@ max_parallel_hazard_walker(Node *node, max_parallel_hazard_context *context)
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return true;
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}
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1171-
if (IsA(node,NextValueExpr))
1171+
elseif (IsA(node,NextValueExpr))
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{
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if (max_parallel_hazard_test(PROPARALLEL_UNSAFE,context))
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return true;
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}
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/*
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* Treat window functions as parallel-restricted because we aren't sure
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* whether the input row ordering is fully deterministic, and the output
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* of window functions might vary across workers if not. (In some cases,
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* like where the window frame orders by a primary key, we could relax
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* this restriction. But it doesn't currently seem worth expending extra
1183+
* effort to do so.)
1184+
*/
1185+
elseif (IsA(node,WindowFunc))
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{
1187+
if (max_parallel_hazard_test(PROPARALLEL_RESTRICTED,context))
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return true;
1189+
}
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/*
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* As a notational convenience for callers, look through RestrictInfo.
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*/

‎src/test/regress/expected/select_parallel.out

Lines changed: 26 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -573,6 +573,32 @@ explain (costs off)
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Index Cond: (unique1 = 1)
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(5 rows)
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-- Window function calculation can't be pushed to workers.
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explain (costs off, verbose)
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select count(*) from tenk1 a where (unique1, two) in
579+
(select unique1, row_number() over() from tenk1 b);
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QUERY PLAN
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----------------------------------------------------------------------------------------------
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Aggregate
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Output: count(*)
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-> Hash Semi Join
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Hash Cond: ((a.unique1 = b.unique1) AND (a.two = (row_number() OVER (?))))
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-> Gather
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Output: a.unique1, a.two
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Workers Planned: 4
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-> Parallel Seq Scan on public.tenk1 a
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Output: a.unique1, a.two
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-> Hash
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Output: b.unique1, (row_number() OVER (?))
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-> WindowAgg
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Output: b.unique1, row_number() OVER (?)
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-> Gather
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Output: b.unique1
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Workers Planned: 4
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-> Parallel Index Only Scan using tenk1_unique1 on public.tenk1 b
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Output: b.unique1
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(18 rows)
601+
576602
-- to increase the parallel query test coverage
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EXPLAIN (analyze, timing off, summary off, costs off) SELECT * FROM tenk1;
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QUERY PLAN

‎src/test/regress/sql/select_parallel.sql

Lines changed: 6 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -204,6 +204,12 @@ set force_parallel_mode=1;
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explain (costs off)
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select stringu1::int2from tenk1where unique1=1;
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-- Window function calculation can't be pushed to workers.
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explain (costs off, verbose)
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selectcount(*)from tenk1 awhere (unique1, two)in
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(select unique1, row_number() over()from tenk1 b);
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212+
207213
-- to increase the parallel query test coverage
208214
EXPLAIN (analyze, timing off, summary off, costs off)SELECT*FROM tenk1;
209215

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