Some memory not freed at the exit of RelationBuildPartitionDesc()

Hi,

In RelationBuildPartitionDesc(), a memory space that use to gather
partitioning
bound info wasn't free at the end. This might not a problem because this
allocated memory will eventually be recovered when the top-level context is
freed, but the case when a partitioned table having 1000s or more
partitions and
this partitioned relation open & close, and its cached entry invalidated in
loop
then we'll have too may call to RelationBuildPartitionDesc() which will keep
wasting some space with every loop.

For a demonstration purpose, I did the following changes to
heap_drop_with_catalog() and tried to drop a partitioned table having 5000
partitions(attached create script) which hit OOM on a machine in no time:

diff --git a/src/backend/catalog/heap.c b/src/backend/catalog/heap.c
index b7bcdd9d0f..6b7bc0d7ae 100644
--- a/src/backend/catalog/heap.c
+++ b/src/backend/catalog/heap.c
@@ -1842,6 +1842,8 @@ heap_drop_with_catalog(Oid relid)
parentOid = get_partition_parent(relid);
LockRelationOid(parentOid, AccessExclusiveLock);

+ rel = relation_open(parentOid, NoLock);
+ relation_close(rel, NoLock);
/*
* If this is not the default partition, dropping it will change the
* default partition's partition constraint, so we must lock it.

I think we should do all partitioned bound information gathering and
calculation in temporary memory context which can be released at the end of
RelationBuildPartitionDesc(), thoughts/Comments?

I did the same in the attached patch and the aforesaid OOM issue is
disappeared.

Regards,
Amul