Re: Performance on inserts

> Bruce Momjian <pgman@candle.pha.pa.us> writes:
> > What I am more concerned about is a join that uses the most common
> > value.  We do an index scan in that case.
> 
> No, we do whichever plan looks cheapest.  Again, it's all about
> statistics.
> 
> Right now, eqjoinsel() is just a stub that returns a constant
> selectivity estimate.  It might be useful to compute some more
> sophisticated value based on pg_statistic entries for the two
> columns, but right now I doubt you could tell much.  Should keep
> the join case in mind when we extend the statistics...

OK, let me be more specific. Suppose the most common value in a column
is 3.  For a query "col = 3", we know 3 is most common, and use the most
common statistics rather than the dispersion statistic, right?

OK, let's assume use of the most common statistic causes a sequential
scan, but use of dispersion causes an index scan.

The query "col = 3" uses sequential scan. In the query "col = tab2.col2",
the dispersion statistic is used, causing an index scan.  

However, assume tab2.col2 equals 3.  I assume this would cause an index
scan because the executor doesn't know about the most common value,
right? Is it worth trying to improve that?

--  Bruce Momjian                        |  http://candle.pha.pa.us pgman@candle.pha.pa.us               |  (610)
853-3000+  If your life is a hard drive,     |  830 Blythe Avenue +  Christ can be your backup.        |  Drexel Hill,
Pennsylvania19026