FAQ

On 3 June 2013 19:41, Jim Nasby wrote:
On 6/2/13 4:45 AM, Simon Riggs wrote:

Will this add too much cost where it doesn't help? I don't know what to
predict there. There's the obvious case of trivial transactions with no
more
than one referential integrity check per FK, but there's also the case
of a
transaction with many FK checks all searching different keys. If the
hash hit
rate (key duplication rate) is low, the hash can consume considerably
more
memory than the trigger queue without preventing many RI queries. What
sort
of heuristic could we use to avoid pessimizing such cases?
I've struggled with that for a while now. Probably all we can say is
that there might be one, and if there is not, then manual decoration
of the transaction will be the way to go.

Just an idea... each backend could keep a store that indicates what FKs this
would help with. For example, any time we hit a transaction that exercises
the same FK more than once, we stick the OID of the FK constraint (or maybe
of the two tables) into a hash that's in that backend's top memory context.
(Or if we want to be real fancy, shared mem).
Yes, that principle would work. We could just store that on the
relcache entry for a table.

It requires a little bookkeeping to implement that heuristic. I'm sure
other ways exist as well.

--
  Simon Riggs http://www.2ndQuadrant.com/
  PostgreSQL Development, 24x7 Support, Training & Services

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postedJun 1, '13 at 8:41a
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