Re: [HACKERS] Re: pgsql: Avoid extra locks in GetSnapshotData if old_snapshot_threshold <

On Fri, May 6, 2016 at 8:28 PM, Kevin Grittner <kgrittn@gmail.com> wrote:
> On Fri, May 6, 2016 at 7:48 PM, Andres Freund <andres@anarazel.de> wrote:

>> That comment reminds me of a question I had: Did you consider the effect
>> of this patch on analyze? It uses a snapshot, and by memory you've not
>> built in a defense against analyze being cancelled.
>
> Will need to check on that.

With a 1min threshold, after loading a table "v" with a million
rows, beginning a repeatable read transaction on a different
connection and opening a cursor against that table, deleting almost
all rows on the original connection, and waiting a few minutes I
see this in the open transaction:

test=# analyze verbose v;
INFO:  analyzing "public.v"
INFO:  "v": scanned 4425 of 4425 pages, containing 1999 live rows and
0 dead rows; 1999 rows in sample, 1999 estimated total rows
ANALYZE
test=# select count(*) from v;
ERROR:  snapshot too old

Meanwhile, no errors appeared in the log from autovacuum.

--
Kevin Grittner
EDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company