Support GiST index support functions that want to cache data across calls.

pg_trgm was already doing this unofficially, but the implementation hadn't
been thought through very well and leaked memory.  Restructure the core
GiST code so that it actually works, and document it.  Ordinarily this
would have required an extra memory context creation/destruction for each
GiST index search, but I was able to avoid that in the normal case of a
non-rescanned search by finessing the handling of the RBTree.  It used to
have its own context always, but now shares a context with the
scan-lifespan data structures, unless there is more than one rescan call.
This should make the added overhead unnoticeable in typical cases.

1 files changed, 2 insertions, 2 deletions

diff --git a/src/backend/access/gist/gistget.c b/src/backend/access/gist/gistget.c
index 60116dfd46..b565d09b38 100644
--- a/src/backend/access/gist/gistget.c
+++ b/src/backend/access/gist/gistget.c
@@ -307,12 +307,12 @@ gistScanPage(IndexScanDesc scan, GISTSearchItem *pageItem, double *myDistances,
 		 * Must call gistindex_keytest in tempCxt, and clean up any leftover
 		 * junk afterward.
 		 */
-		oldcxt = MemoryContextSwitchTo(so->tempCxt);
+		oldcxt = MemoryContextSwitchTo(so->giststate->tempCxt);
 
 		match = gistindex_keytest(scan, it, page, i, &recheck);
 
 		MemoryContextSwitchTo(oldcxt);
-		MemoryContextReset(so->tempCxt);
+		MemoryContextReset(so->giststate->tempCxt);
 
 		/* Ignore tuple if it doesn't match */
 		if (!match)