Re: pgsql: Make decompilation of optimized CASE constructs more robust.

On 27.05.2011 02:44, Tom Lane wrote:
> Make decompilation of optimized CASE constructs more robust.
>
> We had some hacks in ruleutils.c to cope with various odd transformations
> that the optimizer could do on a CASE foo WHEN "CaseTestExpr = RHS" clause.
> However, the fundamental impossibility of covering all cases was exposed
> by Heikki, who pointed out that the "=" operator could get replaced by an
> inlined SQL function, which could contain nearly anything at all. So give
> up on the hacks and just print the expression as-is if we fail to recognize
> it as "CaseTestExpr = RHS". (We must cover that case so that decompiled
> rules print correctly; but we are not under any obligation to make EXPLAIN
> output be 100% valid SQL in all cases, and already could not do so in some
> other cases.) This approach requires that we have some printable
> representation of the CaseTestExpr node type; I used "CASE_TEST_EXPR".

I'm afraid this is still wrong. The "=" OpExpr might also get replaced
with another OpExpr as part of inlining. For example:

-- Equality operator between int4 and bool, where 0 is false and
anything else is true.
postgres=# CREATE FUNCTION int4_bool_eq (int4, bool) returns boolean AS
$$ SELECT ($1 <> 0 AND $2) OR ($1 = 0 AND NOT $2) $$ language sql;
CREATE FUNCTION
postgres=# CREATE OPERATOR = (procedure = int4_bool_eq, leftarg = int4,
rightarg = bool);
CREATE OPERATOR
postgres=# explain verbose SELECT CASE i WHEN true THEN 'yes' ELSE 'no'
END FROM foo;
QUERY PLAN
--------------------------------------------------------------
Seq Scan on public.foo (cost=0.00..40.00 rows=2400 width=4)
Output: CASE i WHEN 0 THEN 'yes'::text ELSE 'no'::text END
(2 rows)

The deparsed CASE statement is backwards. The WHEN expression got
inlined into "CaseTestExpr <> 0", and ruleutils.c incorrectly recognizes
that as the special OpExpr form, and shortens it into "CASE i WHEN 0
...". Seems that we need to also check that the OpExpr uses "=".

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com