Pryscila B Guttoski wrote:
On my master course, I'm studying the PostgreSQL's optimizer.
I don't know if anyone in this list have been participated from the
PostgreSQL's Optimizer development, but maybe someone can help me on this
question.
pgsql-hackers might be more appropriate.
PostgreSQL generates all possible plans of executing the query (using an
almost exhaustive search), then gives a cost to each plan and finally the
cheapest one is selected for execution.
There are other methods for query optimization, one of them is based on plan
transformations (for example, using A-Star algorithm) instead of plan
constructions used by PostgreSQL.
Right, the main query planner uses a nearly-exhaustive search. For
queries with many joins (when the cost of an exhaustive search would be
prohibitive), "GEQO" is used, which uses a genetic algorithm to avoid an
exhaustive search of the solution space.
Does anyone know why this method was choosen?
As far as I know, the main planner algorithm is fairly standard and is
mainly different from System R's canonical algorithm in the details,
like whether non-left-deep plans are pruned.
Are there any papers or researches about it?
There are many papers on the System R algorithm and similar techniques,
which should explain the basic motivations for the design. I'm not aware
of any papers specifically on the PostgreSQL query optimizer, although
there have been a few presentations on it:
http://neilc.treehou.se/optimizer.pdf
http://conferences.oreillynet.com/presentations/os2003/lane_tom.pdf
-Neil
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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
