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costing for arbitrary path

From: uwcssa <uwcssa(at)gmail(dot)com>
To: pgsql-hackers(at)postgresql(dot)org
Subject: costing for arbitrary path
Date: 2007-01-04 03:51:58
Message-ID: f2f562510701031951xb122b05j3742d09bd70df44@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
I wish to estimate the total cost for a given path of a query. This
path is a tree consists of nestloop, hashjoin, mergejoin as internal
nodes with seqscan or idxscan at the leaves.

My approach is to cost the path bottom-up, and invoke the
"cost_nestloop", "cost_merge_join" etc.

The problem is that each join method has some value that seems to
change value each time I cost the SAME path.  For example, the
hash-join has "thisbucketsize" varying from time to time. I noticed
this is because the caching the bucket size estimate inside the
RestrictInfo structure. Is there a easy way to cost a give path?

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