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Commit73b06cf

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Avoid taking a new snapshot for an immutable simple expression in plpgsql.
We already used this optimization if the plpgsql function is read-only.But it seems okay to do it even in a read-write function, if theexpression contains only immutable functions/operators. There wouldonly be a change of behavior if an "immutable" called function dependson seeing database updates made during the current plpgsql function.That's enough of a violation of the promise of immutability that anyonewho complains won't have much of a case.The benefits are significant --- for simple cases like while i < 10000000 loop i := i + 1; end loop;I see net performance improvements around 45%. Of course, real-worldcases won't get that much faster, but it ought to be noticeable.At the very least, this removes much of the performance penalty thatused to exist for forgetting to mark a plpgsql function non-volatile.Konstantin Knizhnik, reviewed by Pavel Stehule, cosmetic changes by meDiscussion:https://postgr.es/m/ed9da20e-01aa-d04b-d085-e6c16b14b9d7@postgrespro.ru
1 parentf67b173 commit73b06cf

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2 files changed

+16
-5
lines changed

2 files changed

+16
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lines changed

‎src/pl/plpgsql/src/pl_exec.c

Lines changed: 15 additions & 5 deletions
Original file line numberDiff line numberDiff line change
@@ -6079,6 +6079,7 @@ exec_eval_simple_expr(PLpgSQL_execstate *estate,
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LocalTransactionIdcurlxid=MyProc->lxid;
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CachedPlan*cplan;
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void*save_setup_arg;
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boolneed_snapshot;
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MemoryContextoldcontext;
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/*
@@ -6150,12 +6151,19 @@ exec_eval_simple_expr(PLpgSQL_execstate *estate,
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/*
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* We have to do some of the things SPI_execute_plan would do, in
6153-
* particular advance the snapshot if we are in a non-read-only function.
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* Without this, stable functions within the expression would fail to see
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* updates made so far by our own function.
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* particular push a new snapshot so that stable functions within the
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* expression can see updates made so far by our own function. However,
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* we can skip doing that (and just invoke the expression with the same
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* snapshot passed to our function) in some cases, which is useful because
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* it's quite expensive relative to the cost of a simple expression. We
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* can skip it if the expression contains no stable or volatile functions;
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* immutable functions shouldn't need to see our updates. Also, if this
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* is a read-only function, we haven't made any updates so again it's okay
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* to skip.
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*/
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oldcontext=MemoryContextSwitchTo(get_eval_mcontext(estate));
6158-
if (!estate->readonly_func)
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need_snapshot= (expr->expr_simple_mutable&& !estate->readonly_func);
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if (need_snapshot)
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{
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CommandCounterIncrement();
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PushActiveSnapshot(GetTransactionSnapshot());
@@ -6180,7 +6188,7 @@ exec_eval_simple_expr(PLpgSQL_execstate *estate,
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estate->paramLI->parserSetupArg=save_setup_arg;
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6183-
if (!estate->readonly_func)
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if (need_snapshot)
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PopActiveSnapshot();
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MemoryContextSwitchTo(oldcontext);
@@ -8051,6 +8059,8 @@ exec_save_simple_expr(PLpgSQL_expr *expr, CachedPlan *cplan)
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/* Also stash away the expression result type */
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expr->expr_simple_type=exprType((Node*)tle_expr);
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expr->expr_simple_typmod=exprTypmod((Node*)tle_expr);
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/* We also want to remember if it is immutable or not */
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expr->expr_simple_mutable=contain_mutable_functions((Node*)tle_expr);
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}
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/*

‎src/pl/plpgsql/src/plpgsql.h

Lines changed: 1 addition & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -234,6 +234,7 @@ typedef struct PLpgSQL_expr
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intexpr_simple_generation;/* plancache generation we checked */
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Oidexpr_simple_type;/* result type Oid, if simple */
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int32expr_simple_typmod;/* result typmod, if simple */
237+
boolexpr_simple_mutable;/* true if simple expr is mutable */
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/*
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* if expr is simple AND prepared in current transaction,

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