Re: [PATCH] Allow multiple recursive self-references

Based on Toms feedback, and due to the fact that SQL:2021 forbids
non-linear recursion, version 2 of the patch allows only linear
recursion. Therefore, later SQL committee decisions on non-linear
recursion should not be problematic.

> [LIN] PostgreSQL does not allow multiple references to the recursive
> common table, even if the recursion is LINEAR. Plenty of examples
> of such queries are found in the documentation of established RDBMSs,
> among these IBM Db2 and MS SQL Server:
>
> - https://www.ibm.com/support/knowledgecenter/ssw_ibm_i_73/sqlp/rbafyrecursivequeries.htm#d60691e2455 <https://www.ibm.com/support/knowledgecenter/ssw_ibm_i_73/sqlp/rbafyrecursivequeries.htm#d60691e2455>
> (example "Two tables used for recursion using recursive common
> table expressions")

See the gist at [1] below for SQL code that features multiple (yet linear)
recursive references. A patched PostgreSQL runs this query as expected.

Best wishes,
--Denis and Torsten

[1] https://gist.github.com/kryonix/73d77d3eaa5a15b3a4bdb7d590fa1253 <https://gist.github.com/kryonix/73d77d3eaa5a15b3a4bdb7d590fa1253>