Re: Vacuum Full - Questions

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(at)gmail(dot)com> wrote:

>
> On Thu, Sep 1, 2016 at 10:32 AM, Patrick B <patrickbakerbr(at)gmail(dot)com>
> wrote:
>
>>
>>
>> 2016-09-01 11:53 GMT+12:00 Venkata B Nagothi <nag1010(at)gmail(dot)com>:
>>
>>>
>>> On Thu, Sep 1, 2016 at 8:41 AM, Patrick B <patrickbakerbr(at)gmail(dot)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
>
>