Thread: Unused CTE affects result set

Dear Postgres community,

I'm seeing unintuitive behavior with a query structured as follows:

WITH used_cte (SELECT a, b, c FROM t1 ...)

, unused_cte (SELECT d, e, f FROM t2 ...)

SELECT * FROM used_cte ...

-- returns no results

WITH used_cte (SELECT a, b, c FROM t1 ...)

-- , unused_cte (SELECT d, e, f FROM t2 ...)

SELECT * FROM used_cte ...

-- returns results

I don't understand why the presence of a CTE that ends up not being included in the final query affects the result set.  Could this be a bug, or is there something I don't understand?

I would say it has to do with what '...' is doing?

In other words I don't see this behavior here for a simple case.

PostgreSQL 14.9 (Ubuntu 14.9-0ubuntu0.22.04.1) on x86_64-pc-linux-gnu, compiled by gcc (Ubuntu 11.4.0-1ubuntu1~22.04) 11.4.0, 64-bit

Thanks,

Gabriel

Adrian Klaver <adrian.klaver@aklaver.com> writes:
> On 11/17/23 10:30 AM, Gabriel Sánchez wrote:
>> I don't understand why the presence of a CTE that ends up not being 
>> included in the final query affects the result set. Could this be a 
>> bug, or is there something I don't understand?

> I would say it has to do with what '...' is doing?

We'd need to see a complete example to do more than speculate.
The planner definitely does skip unreferenced SELECT CTEs, though.

            regards, tom lane