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Commit59b71c6

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Fix handling of NULLs returned by aggregate combine functions.
When strict aggregate combine functions, used in multi-stage/parallelaggregation, returned NULL, we didn't check for that, invoking thecombine function with NULL the next round, despite it being strict.The equivalent code invoking normal transition functions has a checkfor that situation, which did not get copied ina7de3dc. Fix thebug by adding the equivalent check.Based on a quick look I could not find any strict combine functions incore actually returning NULL, and it doesn't seem very likely externalusers have done so. So this isn't likely to have caused issues inpractice.Add tests verifying transition / combine functions returning NULL istested.Reported-By: Andres FreundAuthor: Andres FreundDiscussion:https://postgr.es/m/20171121033642.7xvmjqrl4jdaaat3@alap3.anarazel.deBackpatch: 9.6, where parallel aggregation was introduced
1 parent07bd77b commit59b71c6

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3 files changed

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3 files changed

+145
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lines changed

‎src/backend/executor/nodeAgg.c

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Original file line numberDiff line numberDiff line change
@@ -1246,6 +1246,17 @@ advance_combine_function(AggState *aggstate,
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pergroupstate->noTransValue= false;
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return;
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}
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if (pergroupstate->transValueIsNull)
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{
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/*
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* Don't call a strict function with NULL inputs. Note it is
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* possible to get here despite the above tests, if the combinefn
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* is strict *and* returned a NULL on a prior cycle. If that
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* happens we will propagate the NULL all the way to the end.
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*/
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return;
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}
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}
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/* We run the combine functions in per-input-tuple memory context */

‎src/test/regress/expected/aggregates.out

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Original file line numberDiff line numberDiff line change
@@ -1993,3 +1993,74 @@ NOTICE: sum_transfn called with 4
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(1 row)
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rollback;
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-- test that the aggregate transition logic correctly handles
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-- transition / combine functions returning NULL
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-- First test the case of a normal transition function returning NULL
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BEGIN;
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CREATE FUNCTION balkifnull(int8, int4)
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RETURNS int8
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STRICT
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LANGUAGE plpgsql AS $$
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BEGIN
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IF $1 IS NULL THEN
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RAISE 'erroneously called with NULL argument';
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END IF;
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RETURN NULL;
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END$$;
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CREATE AGGREGATE balk(
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BASETYPE = int4,
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SFUNC = balkifnull(int8, int4),
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STYPE = int8,
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"PARALLEL" = SAFE,
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INITCOND = '0');
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SELECT balk(1) FROM tenk1;
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balk
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------
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(1 row)
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ROLLBACK;
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-- Secondly test the case of a parallel aggregate combiner function
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-- returning NULL. For that use normal transition function, but a
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-- combiner function returning NULL.
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BEGIN ISOLATION LEVEL REPEATABLE READ;
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CREATE FUNCTION balkifnull(int8, int8)
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RETURNS int8
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PARALLEL SAFE
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STRICT
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LANGUAGE plpgsql AS $$
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BEGIN
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IF $1 IS NULL THEN
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RAISE 'erroneously called with NULL argument';
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END IF;
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RETURN NULL;
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END$$;
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CREATE AGGREGATE balk(
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BASETYPE = int4,
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SFUNC = int4_sum(int8, int4),
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STYPE = int8,
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COMBINEFUNC = balkifnull(int8, int8),
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"PARALLEL" = SAFE,
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INITCOND = '0'
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);
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-- force use of parallelism
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ALTER TABLE tenk1 set (parallel_workers = 4);
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SET LOCAL parallel_setup_cost=0;
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SET LOCAL max_parallel_workers_per_gather=4;
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EXPLAIN (COSTS OFF) SELECT balk(1) FROM tenk1;
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QUERY PLAN
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--------------------------------------------------------------------------------
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Finalize Aggregate
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-> Gather
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Workers Planned: 4
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-> Partial Aggregate
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-> Parallel Index Only Scan using tenk1_thous_tenthous on tenk1
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(5 rows)
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SELECT balk(1) FROM tenk1;
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balk
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------
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(1 row)
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ROLLBACK;

‎src/test/regress/sql/aggregates.sql

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Original file line numberDiff line numberDiff line change
@@ -843,3 +843,66 @@ create aggregate my_half_sum(int4)
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select my_sum(one),my_half_sum(one)from (values(1),(2),(3),(4)) t(one);
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rollback;
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-- test that the aggregate transition logic correctly handles
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-- transition / combine functions returning NULL
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-- First test the case of a normal transition function returning NULL
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BEGIN;
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CREATEFUNCTIONbalkifnull(int8, int4)
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RETURNS int8
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STRICT
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LANGUAGE plpgsqlAS $$
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BEGIN
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IF $1 ISNULL THEN
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RAISE'erroneously called with NULL argument';
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END IF;
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RETURNNULL;
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END$$;
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CREATEAGGREGATEbalk(
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BASETYPE= int4,
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SFUNC= balkifnull(int8, int4),
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STYPE= int8,
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"PARALLEL"= SAFE,
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INITCOND='0');
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SELECT balk(1)FROM tenk1;
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ROLLBACK;
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-- Secondly test the case of a parallel aggregate combiner function
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-- returning NULL. For that use normal transition function, but a
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-- combiner function returning NULL.
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BEGIN ISOLATION LEVEL REPEATABLE READ;
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CREATEFUNCTIONbalkifnull(int8, int8)
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RETURNS int8
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PARALLEL SAFE
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STRICT
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LANGUAGE plpgsqlAS $$
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BEGIN
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IF $1 ISNULL THEN
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RAISE'erroneously called with NULL argument';
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END IF;
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RETURNNULL;
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END$$;
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CREATEAGGREGATEbalk(
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BASETYPE= int4,
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SFUNC= int4_sum(int8, int4),
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STYPE= int8,
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COMBINEFUNC= balkifnull(int8, int8),
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"PARALLEL"= SAFE,
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INITCOND='0'
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);
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-- force use of parallelism
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ALTERTABLE tenk1set (parallel_workers=4);
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SET LOCAL parallel_setup_cost=0;
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SET LOCAL max_parallel_workers_per_gather=4;
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EXPLAIN (COSTS OFF)SELECT balk(1)FROM tenk1;
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SELECT balk(1)FROM tenk1;
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ROLLBACK;

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