Question

I’ve come across a query in an application that I’ve inherited that looks like this:

Select * From foo where     1 <> 1 

As I parse that, it should return nothing (1 <> 1 should evaluate to false, right). However (at least on my Oracle box) it comes back with a full listing of everything in foo. When I try the same thing in MSAccess/Jet and MSSQL I get the behaviour I expect. Why is it different for Oracle (and why would the original developer want to do this)?

Note: I’ve seen some superstition about the +s and -s of using ‘where 1 = 1’, and it causing full table scans; but I don’t think this is what the original developer was intending.

Small Update:
In this case foo is a view. When I try the same thing on on an actual table, I get what I would expect (no rows).

Update 2:
I’ve following the code further down the rabbit hole and determined that all he’s doing is trying to grab the field/column names. I’m still at a loss as to why it’s returning the full record set; but only on views.

Literally, he’s building the query in a string and passing it on for another function to execute unaltered.

'VB6 strSQL = 'SELECT * FROM ' & strTableName & ' WHERE 1 <> 1' 

In this case strTableName contains the name of a view.

Update 3:
For reference, here is one of the views I’m having problems with (I’ve changed the field/table/schema names)

CREATE OR REPLACE FORCE VIEW scott.foo (field1,                                         field2,                                         field4,                                         field5,                                         field12,                                         field8,                                         field6,                                         field7,                                         field16,                                         field11,                                         field13,                                         field14,                                         field15,                                         field17                                        ) AS    SELECT   bar.field1,             bar.field2,             DECODE                (yadda.field9, NULL, 'N',                 DECODE (yadda.field3, NULL, 'Y', 'N')                ) AS field4,             bar.field5,             snafu.field6,             DECODE                 (snafu.field6,                  NULL,                 bar.field8,                    bar.field8                  - snafu.field6                 ) AS field7,             DECODE                (yadda.field10,                 NULL,             bar.field12,                 yadda.field10                ) AS field11,             DECODE                (SIGN (  yadda.field10 - bar.field12),                 NULL, 'N', 1, 'N', 0, 'N', -1, 'Y'                ) AS field13,             bar.field14,             ADD_MONTHS                (DECODE (yadda.field10, NULL, bar.field12, yadda.field10                        ),                 bar.field14 * 12                ) AS field15,        FROM clbuttic,             bar,             yadda,             snafu       WHERE clbuttic.asset_type = bar.asset_type         AND bar.field16 = yadda.field9(+)         AND bar.field1 = snafu.field1(+)         AND (bar.field17 IS NULL)    ; 

Appending Order By 1 (or some column name in the select on foo) seems to convince Oracle to give me back the empty set. It’s a long term solution, but not a short term one (changing he code and redeploying is a major PITA). I’m hoping there’s a little known setting on the DB side or something wrong in the View that is the cause of this odd behaviour.

Answer 1

It definitely looks like a bug in the view merging code of the Oracle optimizer. I bet you only get this with views which contain outer joins. Your ORDER BY solves it, because it practically forces a NO_MERGE on the view.

I wouldn’t put either an ORDER BY or a NO_MERGE hint inside the view though, because (depending on your data volume) it could degrade performance of other queries which use the view. You should put a no_merge hint in the outer query:

Select /*+ NO_MERGE(foo) */ * From foo where     1 <> 1 

You should also raise an SR with Oracle support, as this is definitely a bug. That query should never ever return any rows no matter what you are selecting from, or how complex it is inside. Never ever.

I couldn’t reproduce it, so it’s probably fixed in the version I’m using. What’s the db version you are using?