Feature Requests item #2926187, was opened at 2010-01-05 12:45
Message generated for change (Comment added) made by mlkersten
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Category: SQL - general
Group: None
Status: Open
Priority: 5
Private: No
Submitted By: Stefan de Konink (skinkie)
Assigned to: Nobody/Anonymous (nobody)
Summary: SQL: increase cost of a like operation
Initial Comment:
It seems in a query like:
explain select kvk from kvk where length('hello') = length(bedrijfsnaam) and
bedrijfsnaam like '%hello%';
The like operation is before the length, I have attached the mal explain of it.
This is the plan:
project (
| select (
| | table(sys.kvk) [ kvk.kvk, kvk.bedrijfsnaam, kvk.%TID% NOT NULL ]
| ) [ length(kvk.bedrijfsnaam) = length('hello'), convert(kvk.bedrijfsnaam)
like convert('%hello%') ]
) [ kvk.kvk ]
If like is indeed before the length equality function (it seems to be finished
after the 2nd barrier), shouldn't the cost for the like operation be increased?
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>Comment By: Martin Kersten (mlkersten)
Date: 2010-01-05 13:29
Message:
Optimizers generally do not have any knowlegde about the cost of
(user-defined) functions. MonetDB does not use a cost-based reordering of
the plan either to benefit from this specific case.
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