Re: FDW handling count(*) through AnalyzeForeignTable or other constant time push-down

On 25 February 2016 at 09:48, Gabe F. Rudy <rudy(at)goldenhelix(dot)com> wrote:

> Hey all,
>
>
>
> I’m building a FDW around a column-store backend (similar to CStore but
> for genomic data!).
>
>
>
> I have tables in the billions of rows, and have a common query pattern of
> asking for the table size (i.e. SELECT COUNT(*) FROM big_fdw_table; ).
>
>
>
> This is a read-optimized system in which I know in constant time the exact
> dimensions of the table.
>
>
>
> Is there any way to convince Postgres FDW to leverage the analyze row
> counts or even the “double* totalRowCount” returned from the
> AcquireSampleRows callback from my AnalyzeForeignTable function so that it
> does not do a full-table scan for a COUNT(*) etc?
>
>
>
> My current fallback is to export a specialized function that returns the
> table row count for a given FDW table, but that then leaks into the
> user-application driving these queries.
>

Look at TABLESAMPLE, which does mostly what you're asking.

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