I tried this in 6.5beta, and it worked:
select relname from pg_class where relname in
(select relname from pg_class where relname in
(select relname from pg_class)
)
union select relname from pg_class;
>
> ============================================================================
> POSTGRESQL BUG REPORT TEMPLATE
> ============================================================================
>
>
> Your name : Joerg Fischer
> Your email address : fischer@zkm.de
>
> Category : runtime: back-end: SQL
> Severity : serious
>
> Summary: Query processing seems broken
>
> System Configuration
> --------------------
> Operating System : SuSE Linux 6.0 (i386) - Kernel 2.0.36 (ELF)
>
> PostgreSQL version : 6.5beta1
>
> Compiler used : egcs-2.91.60
>
> Hardware:
> ---------
> Pentium II-450, 128 MB RAM, 9 GB U2W HD, Adaptec U2W Cont.
>
> Linux salon09 2.0.36 #3 Mon Apr 26 20:44:52 CEST 1999 i686 unknown
>
>
> Versions of other tools:
> ------------------------
>
>
> --------------------------------------------------------------------------
>
> Problem Description:
> --------------------
>
>
> --------------------------------------------------------------------------
>
> Test Case:
> ----------
> The following query fails with PGSQL 6.5beta1,
> even though it seems correct:
>
> select superext as name from OM_ExtendInheritance where subext in (
> select superext as name from OM_ExtendInheritance where subext in (
> select 'Patient' as name
> )
> union select 'Patient' as name
> );
>
> Postgresql reports:
> ERROR: parser: parse error at or near "union"
>
>
> Could there be a problem handling nested expressions?
>
>
>
> --------------------------------------------------------------------------
>
> Solution:
> ---------
>
>
> --------------------------------------------------------------------------
>
>
>
--
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