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(dot)l(dot)pierson(at)irs(dot)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
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