Re: RFC: planner statistics in 7.2

At 20:48 19/04/01 -0400, Tom Lane wrote:
>
>> This sounds great; can the same be done for clustering. ie. pick a random
>> sample of index nodes, look at the record pointers and so determine how
>> well clustered the table is?
>
>My intention was to use the same tuples sampled for the data histograms
>to estimate how well sorted the data is. However it's not immediately
>clear that that'll give a trustworthy estimate; I'm still studying it ...

I'm not sure you want to know how well sorted it is in general, but you do
want to know the expected cost in IOs of reading all records from a given
index node, so you can more accurately estimate indexscan costs. AFAICS it
does not require that the entire table be sorted. So checking the pointers
on the index nodes gives an idea of clustering.

>
>> ALTER STATISTICS FOR tab[.column] COLLECT n
>> ALTER STATISTICS FOR tab SAMPLE m
>
>Is that more standard than the other syntax?
>

Not at all. It just avoids messing with one of the standard statements.

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