Hybrid Hash/Nested Loop joins and caching results from subplans

Hi, David:
For nodeResultCache.c :

+#define SH_EQUAL(tb, a, b) ResultCacheHash_equal(tb, a, b) == 0

I think it would be safer if the comparison is enclosed in parentheses (in
case the macro appears in composite condition).

+ResultCacheHash_equal(struct resultcache_hash *tb, const ResultCacheKey
*key1,
+ const ResultCacheKey *key2)

Since key2 is not used, maybe name it unused_key ?

+ /* Make a guess at a good size when we're not given a valid size. */
+ if (size == 0)
+ size = 1024;

Should the default size be logged ?

+ /* Update the memory accounting */
+ rcstate->mem_used -= freed_mem;

Maybe add an assertion that mem_used is >= 0 after the decrement (there is
an assertion in remove_cache_entry however, that assertion is after another
decrement).

+ * 'specialkey', if not NULL, causes the function to return false if the
entry
+ * entry which the key belongs to is removed from the cache.

duplicate entry (one at the end of first line and one at the beginning of
second line).

For cache_lookup(), new key is allocated before checking
whether rcstate->mem_used > rcstate->mem_upperlimit. It seems new entries
should probably have the same size.
Can we check whether upper limit is crossed (assuming the addition of new
entry) before allocating new entry ?

+ if (unlikely(!cache_reduce_memory(rcstate, key)))
+ return NULL;

Does the new entry need to be released in the above case?

Cheers