Обсуждение: Re: [EXT] Re: Improve "select count(*)" query - takes more than 30 mins for some large tables

On Tue, 2022-07-12 at 14:25 -0400, MichaelDBA Vitale wrote: 
> On 07/12/2022 2:13 PM Pierson Patricia L (Contractor) <patricia.l.pierson@irs.gov> wrote: 
> > Do a count on the primary key.  Will force index access and you don’t access the entire row which may be very
long.
> > LIKE : select count(ID) from my_table;
> 
> That is not true: doing the select on the primary key will still result in a table scan,
> not an index scan.  The heap always gets accessed for select counts. 

I'd say that both statements are wrong:

- count(id) is *slower* than count(*), because it has to check each "id" if it is
  NULL or not (NULL values are not counted).  count(*) is just the SQL standard's
  weird way of writing a parameterless aggregate; it has nothing to do with the *
  in "SELECT * FROM ".

- Both "SELECT count(id) FROM tab" and "SELECT count(*) FROM tab" can result in an
  index-only scan.  You just need the table to be recently VACUUMed, you need
  a table that is wide enough that a sequential scan is actually slower than an
  index-only scan, and perhaps you need "random_page_cost" to be low enough.

Yours,
Laurenz Albe
-- 
Cybertec | https://www.cybertec-postgresql.com

Laurenz,

Oh really? Even if it did an INDEX ONLY SCAN, it would still be slower because it still has to access the heap in a random I/O kinda way  (like I said-->"The heap always gets accessed for select counts") because visibility info is only found in the HEAP not the index.


Laurenz Albe wrote on 7/12/2022 3:13 PM:

I'd say that both statements are wrong:

Regards,

Michael Vitale

Michaeldba@sqlexec.com

703-600-9343 

> On Jul 12, 2022, at 8:12 PM, MichaelDBA <MichaelDBA@sqlexec.com> wrote:
>
> Oh really? Even if it did an INDEX ONLY SCAN, it would still be slower because it still has to access the heap in a
randomI/O kinda way  (like I said-->"The heap always gets accessed for select counts") because visibility info is only
foundin the HEAP not the index. 

This changed in 9.something. There is now a visibility map, which can, for data that hasn't changed recently, greatly
reducethe amount of access required to the heap to determine visibility. 

On 7/12/22 21:28, Scott Ribe wrote:
>> On Jul 12, 2022, at 8:12 PM, MichaelDBA <MichaelDBA@sqlexec.com> wrote:
>>
>> Oh really? Even if it did an INDEX ONLY SCAN, it would still be slower because it still has to access the heap in a
randomI/O kinda way  (like I said-->"The heap always gets accessed for select counts") because visibility info is only
foundin the HEAP not the index.
 
> This changed in 9.something. There is now a visibility map, which can, for data that hasn't changed recently,

How recently is "recently"?  And does VACUUM clean it up?

-- 
Angular momentum makes the world go 'round.