Cache Hash Index meta page.

I have created a patch to cache the meta page of Hash index in
backend-private memory. This is to save reading the meta page buffer every
time when we want to find the bucket page. In “_hash_first” call, we try to
read meta page buffer twice just to make sure bucket is not split after we
found bucket page. With this patch meta page buffer read is not done, if
the bucket is not split after caching the meta page.

Idea is to cache the Meta page data in rd_amcache and store maxbucket
number in hasho_prevblkno of bucket primary page (which will always be NULL
other wise, so reusing it here for this cause!!!). So when we try to do
hash lookup for bucket page if locally cached maxbucket number is greater
than or equal to bucket page's maxbucket number then we can say given
bucket is not split after we have cached the meta page. Hence avoid reading
meta page buffer.

I have attached the benchmark results and perf stats (refer
hash_index_perf_stat_and_benchmarking.odc [sheet 1: perf stats; sheet 2:
Benchmark results). There we can see improvements at higher clients, as
lwlock contentions due to buffer read are more at higher clients. If I
apply the same patch on Amit's concurrent hash index patch [1] we can see
improvements at lower clients also. Amit's patch has removed a heavy weight
page lock which was the bottle neck at lower clients.

[1] Concurrent Hash Indexes <https://commitfest.postgresql.org/10/647/>

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Thanks and Regards
Mithun C Y
EnterpriseDB: http://www.enterprisedb.com