Message-ID: <480D41C3.3070608@mark.mielke.cc>
Date: Mon, 21 Apr 2008 21:39:15 -0400
From: Mark Mielke <mark@mark.mielke.cc>
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To: PFC <lists@peufeu.com>
CC: Francisco Reyes <lists@stringsutils.com>,
	Gregory Stark <stark@enterprisedb.com>,
	Pgsql performance <pgsql-performance@postgresql.org>
Subject: Re: Group by more efficient than distinct?
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	<87ve2f7bel.fsf@oxford.xeocode.com>
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	<cone.1208704536.428347.85914.1000@zoraida.natserv.net>
	<op.t9zif2yocigqcu@apollo13.peufeu.com>
	<480D283E.1060602@mark.mielke.cc>
In-Reply-To: <480D283E.1060602@mark.mielke.cc>
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Mark Mielke wrote:
> 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? :-)

Hmmm... You did say distinct values, so I can see how that would work 
for distinct. What about seq scan + hash join, though? To complete the 
join, wouldn't it need to have a reference to each of the rows to join 
against? If there is 20 distinct values and 200 rows in the small table 
- wouldn't it need 200 references to be stored?

Cheers,
mark

-- 
Mark Mielke <mark@mielke.cc>