From 1c1cbe279b3c6e8038c8f8079402f069bb4cea4c Mon Sep 17 00:00:00 2001 From: Tom Lane Date: Mon, 4 Jan 2021 12:39:27 -0500 Subject: Rethink the "read/write parameter" mechanism in pl/pgsql. Performance issues with the preceding patch to re-implement array element assignment within pl/pgsql led me to realize that the read/write parameter mechanism is misdesigned. Instead of requiring the assignment source expression to be such that *all* its references to the target variable could be passed as R/W, we really want to identify *one* reference to the target variable to be passed as R/W, allowing any other ones to be passed read/only as they would be by default. As long as the R/W reference is a direct argument to the top-level (hence last to be executed) function in the expression, there is no harm in R/O references being passed to other lower parts of the expression. Nor is there any use-case for more than one argument of the top-level function being R/W. Hence, rewrite that logic to identify one single Param that references the target variable, and make only that Param pass a read/write reference, not any other Params referencing the target variable. Discussion: https://postgr.es/m/4165684.1607707277@sss.pgh.pa.us --- src/backend/utils/adt/arrayfuncs.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-) (limited to 'src/backend/utils/adt/arrayfuncs.c') diff --git a/src/backend/utils/adt/arrayfuncs.c b/src/backend/utils/adt/arrayfuncs.c index 1b618356606..f7012cc5d98 100644 --- a/src/backend/utils/adt/arrayfuncs.c +++ b/src/backend/utils/adt/arrayfuncs.c @@ -2582,8 +2582,11 @@ array_set_element_expanded(Datum arraydatum, /* * Copy new element into array's context, if needed (we assume it's - * already detoasted, so no junk should be created). If we fail further - * down, this memory is leaked, but that's reasonably harmless. + * already detoasted, so no junk should be created). Doing this before + * we've made any significant changes ensures that our behavior is sane + * even when the source is a reference to some element of this same array. + * If we fail further down, this memory is leaked, but that's reasonably + * harmless. */ if (!eah->typbyval && !isNull) { -- cgit v1.2.3