Re: Date Parameter To Query Confusing Optimizer

"By the very definition of a prepared statement the query plan gets stored before the parameter values are known"

 

Is this true for all databases?  It would seem to me that this approach would always lead to the wrong query plan, especially in the case I am testing where the selectivity is very low for the BETWEEN clause I am using.  The trouble is, the BETWEEN clause can also yield a highly selective result--which is basically the case as the database "ages".  See, the date/timestamp column in this case is a good choice as it is a database activity table, a temporal database.  The trouble is at times there is a huge amount of activity, yielding very low selectivity. 

 

Maybe it would be best for me to just disable the indices on the timestamp fields and just use/expect my join would yield the best results.

 

But, back on topic, to me it seems wrong that choice of prepared vs. non-prepared, and protocol 2 vs. 3, would influence the optimizer so profoundly.  I would think it's got to be something I can tune, that prepared statement parameters be considered for execution plan.  Is there such a setting? 

>>> Alban Hertroys <dalroi@solfertje.student.utwente.nl> 1/4/2011 2:21 AM >>>
On 3 Jan 2011, at 23:48, Kurt Westerfeld wrote:

> I have a JDBC-based application which passes date/time parameters using JDBC query parameters, which is performing very badly (ie. doing full table scans).  In an effort to try to narrow down the problem, I am taking the query and running it in interactive SQL mode, but changing the date parameters (which are BETWEEN ? and ? clauses) and placing a date literal instead, using the "date '2011-01-01' syntax.  When I do this, the query runs instantly, obviously using indices on the tables involved.
> 
> Now, I suspect the optimizer is taking the wrong path based on the usage of query parameters.  I'm pretty surprised by this, because I would think the optimizer would do the same thing for any query parameter, however it arrived.  Unfortunately for this situation, the code which forms the query

The problem here is that JDBC uses prepared statements for parameterised queries. By the very definition of a prepared statement the query plan gets stored before the parameter values are known, which forces the database to use a query plan that would work for every possible value of those parameters.

Thus you end up with a generic query plan.

This isn't often a problem, but if a significant number of your possible parameter values exist in a high percentage of your table rows, then chances are you'll end up with a plan with a sequential scan.

You didn't tell what version of Postgres you're using - I recall recent versions (since 8.3?) are smarter about this particular scenario.

Alban Hertroys

--
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