Horton

why do we need two snapshots per query?

Robert Haas <robertmhaas@gmail.com>

View thread
From: Robert Haas <robertmhaas@gmail.com>
To: pgsql-hackers@postgresql.org
Date: 2011-11-11T14:07:33Z
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.

I noticed while playing around this morning that, in read committed
mode, the following query - or any other straightforward query - calls
GetSnapshotData() twice:

SELECT 1;

What happens is this:

exec_simple_query() calls analyze_requires_snapshot() on the query.
Since the query is a SELECT, this returns true, whereupon
exec_simple_query() takes a snapshot for parse analysis / planning. It
then plans the query and releases the snapshot.  exec_simple_query()
then calls CreatePortal(), PortalDefineQuery(), and PortalStart(), the
last of which takes a new snapshot to run the query.

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.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company