On Tue, Jun 12, 2012 at 6:26 PM, Jeff Davis <pg...@j-davis.com> wrote: > On Tue, 2012-06-12 at 18:02 -0400, Tom Lane wrote: >> Or (d) it's not a problem, since the inserting XID is still busy >> according to the readers' snapshots. > > How much of a savings did we get from PD_ALL_VISIBLE when it was added > into the page-at-a-time visibility check? > > >From 608195a3a3656145a7eec7a47d903bc684011d73: > > "In addition to the visibility map, there's a new PD_ALL_VISIBLE flag on > each heap page, also indicating that all tuples on the page are visible > to all transactions. It's important that this flag is kept up-to-date. > It is also used to skip visibility tests in sequential scans, which > gives a small performance gain on seqscans." > > If "small" means that it's something we can give up, then focusing on > HEAP_XMIN_COMMITTED makes sense. But if we can't give it up, then we > need to take it into account in the proposal.
It's significant. rhaas=# create table foo (a int, b text); ERROR: relation "foo" already exists rhaas=# create table bar (a int, b text); CREATE TABLE rhaas=# insert into bar select g, random()::text||random()::text||random()::text||random()::text from generate_series(1,1000000) g; INSERT 0 1000000 rhaas=# \timing Timing is on. rhaas=# select sum(1) from bar; sum --------- 1000000 (1 row) Time: 257.500 ms rhaas=# select sum(1) from bar; sum --------- 1000000 (1 row) Time: 140.763 ms rhaas=# select sum(1) from bar; sum --------- 1000000 (1 row) Time: 142.760 ms rhaas=# select sum(1) from bar; sum --------- 1000000 (1 row) Time: 140.603 ms rhaas=# vacuum bar; VACUUM Time: 133.084 ms rhaas=# select sum(1) from bar; sum --------- 1000000 (1 row) Time: 123.591 ms rhaas=# select sum(1) from bar; sum --------- 1000000 (1 row) Time: 123.096 ms rhaas=# select sum(1) from bar; sum --------- 1000000 (1 row) Time: 122.653 ms So vacuuming to set the PD_ALL_VISIBLE bits is buying us more than 10% here. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
