Re: Cache Hash Index meta page.

On Thu, Jan 26, 2017 at 1:48 PM, Mithun Cy <mithun(dot)cy(at)enterprisedb(dot)com> wrote:
> _v11 API's was self-sustained one but it does not hold pins on the
> metapage buffer. Whereas in _v12 we hold the pin for two consecutive
> reads of metapage. I have taken your advice and producing 2 different
> patches.

Hmm. I think both of these APIs have some advantages. On the one
hand, passing metabuf sometimes allows you to avoid pin/unpin cycles -
e.g. in hashbulkdelete it makes a fair amount of sense to keep the
metabuf pinned once we've had to read it, just in case we need it
again. On the other hand, it's surprising that passing a value for
the metabuf forces the cached to be refreshed. I wonder if a good API
might be something like this:

HashMetaPage _hash_getcachedmetap(Relation rel, Buffer *metabuf, bool
force_refresh);

If the cache is initialized and force_refresh is not true, then this
just returns the cached data, and the metabuf argument isn't used.
Otherwise, if *metabuf == InvalidBuffer, we set *metabuf to point to
the metabuffer, pin and lock it, use it to set the cache, release the
lock, and return with the pin still held. If *metabuf !=
InvalidBuffer, we assume it's pinned and return with it still pinned.

--
Robert Haas
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