Re: Group by more efficient than distinct?

[Mark Mielke <mark(at)mark(dot)mielke(dot)cc>]

PFC wrote:
>     Actually, the memory used by the hash depends on the number of 
> distinct values, not the number of rows which are processed...
>     Consider :
>
> SELECT a GROUP BY a
> SELECT a,count(*) GROUP BY a
>
>     In both cases the hash only holds discinct values. So if you have 
> 1 million rows to process but only 10 distinct values of "a", the hash 
> will only contain those 10 values (and the counts), so it will be very 
> small and fast, it will absorb a huge seq scan without problem. If 
> however, you have (say) 100 million distinct values for a, using a 
> hash would be a bad idea. As usual, divide the size of your RAM by the 
> number of concurrent connections or something.
>     Note that "a" could be a column, several columns, anything, the 
> size of the hash will be proportional to the number of distinct 
> values, ie. the number of rows returned by the query, not the number 
> of rows processed (read) by the query. Same with hash joins etc, 
> that's why when you join a very small table to a large one Postgres 
> likes to use seq scan + hash join on the small table.

This surprises me - hash values are lossy, so it must still need to 
confirm against the real list of values, which at a minimum should 
require references to the rows to check against?

Is PostgreSQL doing something beyond my imagination? :-)

Cheers,
mark

--
Mark Mielke <mark(at)mielke(dot)cc>