Обсуждение: pgbench filler columns
While looking at the compressibility of WAL files generated by pgbench, which is close to 90%, I first thought its because of the "filler" column in the accounts table. But a comment in pgbench.c says:
/*
* Note: TPC-B requires at least 100 bytes per row, and the "filler"
* fields in these table declarations were intended to comply with that.
* But because they default to NULLs, they don't actually take any space.
* We could fix that by giving them non-null default values. However, that
* would completely break comparability of pgbench results with prior
* versions. Since pgbench has never pretended to be fully TPC-B
* compliant anyway, we stick with the historical behavior.
*/
But I find it otherwise. On my machine, accounts table can only fit 62 tuples in a page with default fillfactor. The following queries show that filler column is NOT NULL, but set to empty string. I have tested on 8.2, 8.4 and master and they all show the same behavior. So I don't know if that bug report itself was wrong or if I am reading the comment wrong.
postgres=# select count(*) from pgbench_accounts where filler IS NULL;
count
-------
0
(1 row)
postgres=# select count(*) from pgbench_accounts where filler = '';
count
--------
100000
(1 row)
Thanks,
Pavan
Pavan Deolasee
http://www.linkedin.com/in/pavandeolasee
On Thu, Sep 26, 2013 at 2:05 PM, Pavan Deolasee <pavan.deolasee@gmail.com> wrote:
While looking at the compressibility of WAL files generated by pgbench, which is close to 90%, I first thought its because of the "filler" column in the accounts table. But a comment in pgbench.c says:/** Note: TPC-B requires at least 100 bytes per row, and the "filler"* fields in these table declarations were intended to comply with that.* But because they default to NULLs, they don't actually take any space.* We could fix that by giving them non-null default values. However, that* would completely break comparability of pgbench results with prior* versions. Since pgbench has never pretended to be fully TPC-B* compliant anyway, we stick with the historical behavior.*/The comment about them being NULL and hence not taking up any space is added by commit b7a67c2840f193f in response to this bug report:
On a more careful look, it seems the original bug report complained about all tables except accounts. And all other tables indeed have "filler" as NULL. But the way comment is written it seems as if it applies to all DDLs. Should we just fix the comment and say its applicable for all tables except accounts ?
Thanks,
Pavan
-- Pavan Deolasee
http://www.linkedin.com/in/pavandeolasee
On Thu, Sep 26, 2013 at 03:23:30PM +0530, Pavan Deolasee wrote: > On Thu, Sep 26, 2013 at 2:05 PM, Pavan Deolasee <pavan.deolasee@gmail.com>wrote: > > > While looking at the compressibility of WAL files generated by pgbench, > > which is close to 90%, I first thought its because of the "filler" column > > in the accounts table. But a comment in pgbench.c says: > > > > /* > > * Note: TPC-B requires at least 100 bytes per row, and the "filler" > > * fields in these table declarations were intended to comply with > > that. > > * But because they default to NULLs, they don't actually take any > > space. > > * We could fix that by giving them non-null default values. However, > > that > > * would completely break comparability of pgbench results with prior > > * versions. Since pgbench has never pretended to be fully TPC-B > > * compliant anyway, we stick with the historical behavior. > > */ > > > > The comment about them being NULL and hence not taking up any space is > > added by commit b7a67c2840f193f in response to this bug report: > > > > http://www.postgresql.org/message-id/200710170810.l9H8A76I080568@wwwmaster.postgresql.org > > > > > On a more careful look, it seems the original bug report complained about > all tables except accounts. And all other tables indeed have "filler" as > NULL. But the way comment is written it seems as if it applies to all DDLs. Agreed. > Should we just fix the comment and say its applicable for all tables except > accounts ? Please do. -- Noah Misch EnterpriseDB http://www.enterprisedb.com
On Thu, Sep 26, 2013 at 7:20 PM, Noah Misch <noah@leadboat.com> wrote:
--
Pavan Deolasee
http://www.linkedin.com/in/pavandeolasee
On Thu, Sep 26, 2013 at 03:23:30PM +0530, Pavan Deolasee wrote:Please do.
> Should we just fix the comment and say its applicable for all tables except
> accounts ?
How about something like this ? Patch attached.
Thanks,
Pavan
Pavan Deolasee
http://www.linkedin.com/in/pavandeolasee
Вложения
On Fri, Sep 27, 2013 at 4:03 PM, Pavan Deolasee <pavan.deolasee@gmail.com> wrote: > On Thu, Sep 26, 2013 at 7:20 PM, Noah Misch <noah@leadboat.com> wrote: >> >> On Thu, Sep 26, 2013 at 03:23:30PM +0530, Pavan Deolasee wrote: >> >> > Should we just fix the comment and say its applicable for all tables >> > except >> > accounts ? >> >> Please do. >> > > How about something like this ? Patch attached. Thanks! Committed. Regards, -- Fujii Masao