2011/2/22 PostgreSQL - Hans-Jürgen Schönig <postg...@cybertec.at>:
> how does it work? we try to find suitable statistics for an arbitrary length 
> list of conditions so that the planner can use it directly rather than 
> multiplying all the selectivities. this should make estimates a lot more 
> precise.
> the current approach can be extended to work with expressions and well as 
> "straight" conditions.

/me prepares to go down in flames.

Personally, I think the first thing we ought to do is add a real, bona
fide planner hint to override the selectivity calculation manually,
maybe something like this:

WHERE (x < 5 AND y = 1) SELECTIVITY (0.1);

Then, having provided a method for the DBA to extinguish the raging
flames of searing agony which are consuming them while a crocodile
chews off their leg and their boss asks them why they didn't use
Oracle, we can continue bikeshedding about the best way of fixing this
problem in a more user-transparent fashion.

As to the approach you've proposed here, I'm not sure I understand
what this is actually doing.  Selectivity estimates aren't made
directly for predicates; they're made based on MCV and histogram
information for predicates.

Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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