Fail Fast In CTAS/CMV If Relation Already Exists To Avoid Unnecessary Rewrite, Planning Costs

Hi,

Currently, for CTAS or CREATE MATERIALIZED VIEW(CMV) without
if-not-exists clause, the existence of the relation gets checked
during the execution of the select part and an error is thrown there.
All the unnecessary rewrite and planning for the select part would
have happened just to fail later. However, if if-not-exists clause is
present, then a notice is issued and returned immediately without any
further rewrite or planning for . This seems somewhat inconsistent to
me.

I propose to check the relation existence early in ExecCreateTableAs()
as well as in ExplainOneUtility() and throw an error in case it exists
already to avoid unnecessary rewrite, planning and execution of the
select part.

Attaching a patch. Note that I have not added any test cases as the
existing test cases in create_table.sql and matview.sql would cover
the code.

Thoughts?

With Regards,
Bharath Rupireddy.
EnterpriseDB: http://www.enterprisedb.com