Fix performance bug from conflict between two previous improvements.
My expanded-objects patch (commit
1dc5ebc9077ab742) included code to make
plpgsql pass expanded-object variables as R/W pointers to certain functions
that are trusted for modifying such variables in-place. However, that
optimization got broken by commit
6c82d8d1fdb1f126, which arranged to share
a single ParamListInfo across most expressions evaluated by a plpgsql
function. We don't want a R/W pointer to be passed to other functions
just because we decided one function was safe! Fortunately, the breakage
was in the other direction, of never passing a R/W pointer at all, because
we'd always have pre-initialized the shared array slot with a R/O pointer.
So it was still functionally correct, but we were back to O(N^2)
performance for repeated use of "a := a || x". To fix, force an unshared
param array to be used when the R/W param optimization is active.
Commit
6c82d8d1fdb1f126 is in HEAD only, so no need for a back-patch.