Vacuum Full - Questions

Yes that very well could happen because the size of the table changed, as
well as stats being more accurate now. Just because you have a seq scan
doesn't mean the planer is making a bad choice.

On Thu, Sep 1, 2016 at 8:41 AM, Patrick B <patrickbakerbr@gmail.com> wrote:

Hi guys,

A dev has ran a VACUUM FULL command into our test database running
PostgreSQL 9.5 (I know... goddamn!!!!)...

... after the Vacuum Full, some queries start using SEQ scans instead of
indexes...

Does that happen because of the size of the table? The table that I'm
referring to is 150MB big after the vacuum (Before was 1G)...

Yes, it is possible that sequential scans after vacuum full are cheaper
than Index scans before vacuum full ? do you see improvement in query
response times ?
How does the cost differ ?

Regards,
Venkata B N

Fujitsu Australia

2016-09-01 11:53 GMT+12:00 Venkata B Nagothi <nag1010@gmail.com>:

On Thu, Sep 1, 2016 at 8:41 AM, Patrick B <patrickbakerbr@gmail.com>
wrote:

Hi guys,

A dev has ran a VACUUM FULL command into our test database running
PostgreSQL 9.5 (I know... goddamn!!!!)...

... after the Vacuum Full, some queries start using SEQ scans instead of
indexes...

Does that happen because of the size of the table? The table that I'm
referring to is 150MB big after the vacuum (Before was 1G)...

Yes, it is possible that sequential scans after vacuum full are cheaper
than Index scans before vacuum full ? do you see improvement in query
response times ?
How does the cost differ ?

Regards,
Venkata B N

Fujitsu Australia

Well... the response time was worst than when using index.. that's very
weird... I've re-created the indexes now a ran ANALYZE and the query is
using again the index.. just wanted understand what happened...

On Thu, Sep 1, 2016 at 10:32 AM, Patrick B <patrickbakerbr@gmail.com> wrote:

2016-09-01 11:53 GMT+12:00 Venkata B Nagothi <nag1010@gmail.com>:

On Thu, Sep 1, 2016 at 8:41 AM, Patrick B <patrickbakerbr@gmail.com>
wrote:

Hi guys,

A dev has ran a VACUUM FULL command into our test database running
PostgreSQL 9.5 (I know... goddamn!!!!)...

... after the Vacuum Full, some queries start using SEQ scans instead of
indexes...

Does that happen because of the size of the table? The table that I'm
referring to is 150MB big after the vacuum (Before was 1G)...

Yes, it is possible that sequential scans after vacuum full are cheaper
than Index scans before vacuum full ? do you see improvement in query
response times ?
How does the cost differ ?

Regards,
Venkata B N

Fujitsu Australia

Well... the response time was worst than when using index.. that's very
weird... I've re-created the indexes now a ran ANALYZE and the query is
using again the index.. just wanted understand what happened...

There you go.. Running ANALYZE made the difference. Whenever you execute
VACUUM or VACUUM FULL make sure you execute ANALYZE so that latest stats
are updated. Sorry, i should have mentioned this earlier.

Regards,
Venkata B N

Fujitsu Australia

Hi Patrick,

I believe Vacuum full rebuilds the indexes automatically by default, as a
new copy of the table is created.

Because the indexes are new, no stats are available to the optimiser to
make an informed decision about whether to utilise it or not, so it doesn't.

Once the analyze is performed as Venkata said, the stats are available for
it to make that decision. So, you didn't really need to build the indexes
again.

Hope that makes it a little clearer.

Cheers

On Thu, Sep 1, 2016 at 1:57 PM, Venkata B Nagothi <nag1010@gmail.com> wrote: