On Oct 12, 2006, at 4:26 AM, Andrew Sullivan wrote:
On Thu, Oct 12, 2006 at 08:34:45AM +0200, Florian Weimer wrote:
Some statistics are very hard to gather from a sample, e.g. the
number
of distinct values in a column.
Then how can the DBA know it, either? The problem with this sort of
argument is always that people are claiming some special knowledge is
available to the DBA. If it's true that the DBA really _can_ know
this stuff, then there must be some way to learn it. Which means
that you can, in principle, figure out ways to communicate that to
the optimizer.
Yes, but it may be much more efficient for the human to tell the
computer than for the computer to introspect things. Take, for
example, ndisinct as data grows large. I, the database designer, may
know (or simply see) that a certain foreign key column will have
roughly a certain cardinality regardless of how big the table gets.
It's a lot more efficient for me to tell the system that up front
then have it need to do a full table scan or tens of millions of rows
periodically to figure it out, or worse--as it is currently--to come
up with an estimate that is multiple orders of magnitude off, even
with the stats target turned all the way up.
I realize that this is a case that is possible to do manually now,
sort of. I can tweak the stats table myself. But it would be nice if
you could do it in such a way that it would override what analyze
comes up with on a case-by-case basis.
We could have a perfect query planner, but feed it bad stats and it
will still make poor decisions.
I'm of the strong opinion that hinting the data is much better than
hinting the queries. There tends to be many fewer places you need to
do that, and new queries can automatically take advantage.
I like the suggestion, though, that there be ways to codify known
relationships in the system in such a way that the optimizer can
learn to use that information. _That_ seems to me to be a big
improvement, because it can be taken into consideration along with
relationships that emerge from the statistics, that the DBA may not
know about.
I'm all for things the computer can do for me automagically. It's
just good to have the ability to tell the computer about things you
know about the data that it either can't efficiently figure out or
can't figure out at all.
-Casey
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TIP 9: In versions below 8.0, the planner will ignore your desire to
choose an index scan if your joining column's datatypes do not
match
