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Re: TB-sized databases

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Michael Stone <mstone+postgres(at)mathom(dot)us>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: TB-sized databases
Date: 2007-12-06 16:13:18
Message-ID: 15753.1196957598@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-performance
Michael Stone <mstone+postgres(at)mathom(dot)us> writes:
> OTOH, the planner can really screw up queries on really large databases. 
> IIRC, the planner can use things like unique constraints to get some 
> idea, e.g., of how many rows will result from a join. Unfortunately, 
> the planner can't apply those techniques to certain constructs common in 
> really large db's (e.g., partitioned tables--how do you do a unique 
> constraint on a partitioned table?) I've got some queries that the 
> planner thinks will return on the order of 10^30 rows for that sort of 
> reason. In practice, the query may return 10^3 rows, and the difference 
> between the seq scan and the index scan is the difference between a 
> query that takes a few seconds and a query that I will never run to 
> completion. I know the goal would be to make the planner understand 
> those queries better,

Indeed, and if you've got examples where it's that far off, you should
report them.

			regards, tom lane

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