Horton

Re: why do we need two snapshots per query?

Dimitri Fontaine <dimitri@2ndquadrant.fr>

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From: Dimitri Fontaine <dimitri@2ndQuadrant.fr>
To: Robert Haas <robertmhaas@gmail.com>
Cc: pgsql-hackers@postgresql.org
Date: 2011-11-11T19:21:39Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Fix copyright notices, other minor editing in new range-types code.

  2. Redesign the plancache mechanism for more flexibility and efficiency.

Robert Haas <robertmhaas@gmail.com> writes:
> Considering that GetSnapshotData() is the number-one consumer of CPU
> time on many profiling runs I've done, this seems needlessly
> inefficient.  Can't we arrange to retain the snapshot used for parse
> analysis / planning and reuse it for the portal that we create just
> afterwards?  Off the top of my head, I'm not exactly sure how to do
> that cleanly, but it seems like it should work.

Please refer to this thread:

  http://postgresql.1045698.n5.nabble.com/One-Shot-Plans-td4488820.html

It seems one of the more prominent drawback of Simon's approach to
one-shot plans then was which snapshot it's running against, so your
proposal to optimize one-shot plan by enforcing the use of a single
snapshot looks like a step forward here.

The other problem is how to recognize a query as being a candidate for
one-shot optimization, but I guess exec_simple_query (as opposed to the
v3 protocol) applies.

Regards,
-- 
Dimitri Fontaine
http://2ndQuadrant.fr     PostgreSQL : Expertise, Formation et Support