Re: BUG #15486: PG11 jit on 50x slower than jit off

>>>>> "Andres" == Andres Freund <andres(at)anarazel(dot)de> writes:

Andres> This actually isn't great outside of JIT compilation as well -
Andres> we waste a substantial portion of time rebuilding the
Andres> expression everytime, and the hashtable grows every time as
Andres> well.

Why would the hashtable grow? All of BuildTupleHashTable's callers
follow this pattern: the hashtable is allocated in a memory context that
will be reset on every rescan. Everything the hash table initialization
does is assumed to be completely discardable.

Which means that if you want to add a ResetTupleHashTable, it'll mean
significant reorganization of the callers' use of memory contexts, since
the hashtable will have to go into per-query memory (and presumably
create a child context of its own).

(There's an implicit assumption by all of BuildTupleHashTable's callers
that it will not allocate anything outside of the passed-in tablecxt,
which will be reset. Having BuildTupleHashTable allocate anything in its
caller's context is effectively an API break. But I notice this
_already_ got broken in pg11: the ExprContext that now gets allocated in
BuildTupleHashTable will be leaked into per-query memory on each cycle.)

--
Andrew (irc:RhodiumToad)