Tom Raney <[EMAIL PROTECTED]> writes:
> Alvaro Herrera wrote:
>> Just wondering, wouldn't it be enough to obtain a tuple count estimate
>> by using reltuples / relpages * RelationGetNumberOfBlocks, like the
>> planner does?

> We thought of that and the verdict is still out whether it is more 
> costly to scan the entire relation to get the accurate count or use the 
> estimate and hope for the best with the possibility of splits occurring 
> during the build.   If we use the estimate and it is completely wrong 
> (with the actual tuple count being much higher) the sort will provide no 
> benefit and it will behave as did the original code.

I think this argument is *far* too weak to justify an extra pass over
the relation.  The planner-style calculation is quite unlikely to give a
major underestimate of the rowcount.  It might overestimate, eg if the
relation is bloated by dead tuples, but an error in that direction won't
kill you.

                        regards, tom lane

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