bug or lacking doc hint

Hi,

I have had a perf (++) pb with a join plan cf the pb with join plan thread.
I did simplify the thing up to when its a simple join between a 15M lines
table and a 30k lines table.
if I put in the on part something like table1.a=table2.b, Postgres does the
job in around 1 seconde.

if in the on part of the join I write table1.a=table2.b OR
substr(table1.c,x,x) =table2.d

then I have to cancel the request after 20 minutes seeing disk usage
growing and growing.

When I ask this list, David Rowley suggest to rewrite the SQL, replacing
the OR by a union.

Fine, this do work, even if a bit complex as the original SQL was a set of
intricate joins.

So, either this behaviour ( postgres unable to find an appropriate plan for
join with OR clauses)
is a true bug which is waiting to be corrected, either the doc is lacking a
chapter on which one can read that Postgres does NOT support that syntax as
soon as there is some data volumes.

So, now I am writing some kind of recipe book for the users of that DB.

What should I write ?

Marc MILLAS
Senior Architect
+33607850334
www.mokadb.com