Add an 'enable_material' GUC.

Robert Haas <rhaas@postgresql.org>

Commit: 5b89ef384c7719478bb08b0c771dcbfdc51d507e
Author: Robert Haas <rhaas@postgresql.org>
Date: 2010-04-19T00:55:26Z
Releases: 9.0.0
Add an 'enable_material' GUC.

The logic for determining whether to materialize has been significantly
overhauled for 9.0.  In case there should be any doubt about whether
materialization is a win in any particular case, this should provide a
convenient way of seeing what happens without it; but even with enable_material
turned off, we still materialize in cases where it is required for
correctness.

Thanks to Tom Lane for the review.

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