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

On Fri, Dec 11, 2020 at 03:03:46PM +0530, Bharath Rupireddy wrote:
> I may not have got your above scenario correctly(it will be good if
> you can provide the use case in case I want to check something there).

It is possible to have DML queries in WITH clauses, as long as they
use RETURNING to feed tuples to the outer query. Just imagine
something like that:
=# explain analyze
create table if not exists aa as
with insert_query as
(insert into aa values (1) returning a)
select * from insert_query;

Please note that this case fails with your patch, but the presence of
IF NOT EXISTS should ensure that we don't fail and issue a NOTICE
instead, no? Taking this case specifically (OK, I am playing with
the rules a bit to insert data into the relation itself, still), this
query may finish by adding tuples to the table whose creation should
have been bypassed but the query got executed and inserted tuples.
That's one example of behavior that may be confusing. There may be
others, but it seems to me that it may be simpler to execute or even
plan the query at all if the relation already exists.
--
Michael