Re: Cache Hash Index meta page.

-        bucket = _hash_hashkey2bucket(hashkey,
-                                      metap->hashm_maxbucket,
+        bucket = _hash_hashkey2bucket(hashkey, metap->hashm_maxbucket,
                                       metap->hashm_highmask,
                                       metap->hashm_lowmask);

This hunk appears useless.

+        metap = (HashMetaPage)rel->rd_amcache;

Whitespace.

+        /*  Cache the metapage data for next time*/

Whitespace.

+        /* Check if this bucket is split after we have cached the metapage.

Whitespace.

Shouldn't _hash_doinsert() be using the cache, too?

I think it's probably not a good idea to cache the entire metapage.  The only things that you are "really" trying to cache, I think, are hashm_maxbucket, hashm_lowmask, and hashm_highmask.  The entire HashPageMetaData structure is 696 bytes on my machine, and it doesn't really make sense to copy the whole thing into memory if you only need 16 bytes of it.  It could even be dangerous -- if somebody tries to rely on the cache for some other bit of data and we're not really guaranteeing that it's fresh enough for that.

I'd suggest defining a new structure HashMetaDataCache with members hmc_maxbucket, hmc_lowmask, and hmc_highmask.  The structure can have a comment explaining that we only care about having the data be fresh enough to test whether the bucket mapping we computed for a tuple is still correct, and that for that to be the case we only need to know whether a bucket has suffered a new split since we last refreshed the cache.

The comments in this patch need some work, e.g.:

-
+       oopaque->hasho_prevblkno = maxbucket;

No comment?