BUG #5794: 'explain' fails, but executing sql is ok.

The following bug has been logged online:

Bug reference: 5794
Logged by: Peter Speck
Email address: speck@vitality.dk
PostgreSQL version: 9.0.2
Operating system: Mac OS X Server 10.6.5
Description: 'explain' fails, but executing sql is ok.
Details:

The following SQL succeeds when executed as-is, but when the SELECT is
prepended with 'EXPLAIN', it fails with:

ERROR: XX000: unexpected CASE WHEN clause: 314
LOCATION: get_rule_expr, ruleutils.c:4939

CREATE TABLE z (tag VARCHAR(30)); -- empty

SELECT t.*
FROM (
SELECT
CASE tag
WHEN 'a' THEN 'a'
ELSE
CASE TRUE
WHEN (tag LIKE 'abc%' OR tag LIKE 'def%' ) THEN 'b'
ELSE 'c'
END
END AS stuff
FROM z
) AS t
ORDER BY CASE TRUE WHEN t.stuff LIKE 'ghi%' THEN t.stuff ELSE 'z' END;