> Gaetano,
> > don't you think the best statistic target for a boolean
> > column is something like 2?  Or in general the is useless
> > have a statistics target > data type cardinality ?
> It depends, really, on the proportionality of the boolean values; if they're 
> about equal, I certainly wouldn't raise Stats from the default of 10.   If, 
> however, it's very dispraportionate -- like 2% true and 98% false -- then it 
> may pay to have better statistics so that the planner doesn't assume 50% 
> hits, which it otherwise might.

No, actually the stats table keeps the n most common values and their
frequency (usually in percentage). So really a target of 2 ought to be enough
for boolean values. In fact that's all I see in pg_statistic; I'm assuming
there's a full histogram somewhere but I don't see it. Where would it be?

However the target also dictates how large a sample of the table to take. A
target of two represents a very small sample. So the estimations could be
quite far off.

I ran the experiment and for a table with 2036 false rows out of 204,624 the
estimate was 1720. Not bad. But then I did vacuum full analyze and got an
estimate of 688. Which isn't so good.


---------------------------(end of broadcast)---------------------------
TIP 9: the planner will ignore your desire to choose an index scan if your
      joining column's datatypes do not match

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