Re: [HACKERS] Hypothetical suggestions for planner,

On Mon, 2003-05-05 at 23:25, Tom Lane wrote:
> Josh Berkus <josh(at)agliodbs(dot)com> writes:
> > THE IDEA: The planner should keep statistics on the correlation of foreign
> > keys and apply them to the expected row counts for EXISTS clause limitations,
> > and possibly for other query types as well.
>
> It's a thought. Keeping complete cross-column correlation stats (for
> every combination of columns in the DB) is obviously out of the
> question. If you're gonna do it you need a heuristic to tell you which
> combinations of columns are worth keeping track of --- and foreign-key
> relationships seem like a reasonable guide to the interesting
> combinations.

How about generalizing this into something useful for all queries?

And to make this problem not just guess work on the part of the
optimizer, how about having a separate "backslash command" so that
the DBA can add specific/important/crucial predicates that he needs
optimized.

Thus, if there's a query with a large WHERE clause that has an
optimized predicate inside it, the statistics would be used.

> WHERE event_day BETWEEN '2003-04-08' AND '2003-05-18')

What this sounds to me like, though, is that in order for it to
work quickly, the postmaster would have to keep track in system
tables each value of event_day and it's COUNT(*), thus more I/O
overhead.

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