Mike Ault's thoughts on various topics, Oracle related and not. Note: I reserve the right to delete comments that are not contributing to the overall theme of the BLOG or are insulting or demeaning to anyone. The posts on this blog are provided “as is” with no warranties and confer no rights. The opinions expressed on this site are mine and mine alone, and do not necessarily represent those of my employer.
Thursday, March 23, 2006
WHERE 1=2 and Parallel Isn't
Found out some interesting things yesterday. First, while reviewing a clients Oracle application which had been ported from SQL Server, I noticed they were using the constructs “WHERE 1=1” and “WHERE 1=2” in order to build dynamic SQL for dynamic queries. In the first case of “WHERE 1=1” it was always followed by a group of one or more “AND” clauses so the optimizer would of course practice predicate elimination and remove it. However in the second case, “WHERE 1=2” it was being used to merely return the column headers to avoid error handling.
It is this second case, “WHERE 1=2” I want to talk about. Of course since 1 never equals 2 you never get a result back, however it forces a full table scan! In testing with 10gR1 version 10.1.0.4 and on 9iR2, 18.104.22.168 if there was no bitmap index on the table using the “WHERE 1=2” construct resulted in a full table scan. I tried using a unique and a non-unique B-Tree and they were ignored, seems only a bitmap index would allow the optimizer to determine that 1 would never equal 2, why this is so is a mystery since obviously the bitmap had nothing to do with whether 1=1 or 1=2.
In SQL Server, the optimizer is smart enough to realize 1 will never equal 2 and there, it simply returns the column list without any other action. Oracle, you paying attention? Make a note!
What we ended up doing in this clients case was to replace the “WHERE 1=2” with a clause that equated the primary key of the table with an impossible value for that key, in this case the ID was being passed in as a GUID (a hexadecimal value) so we use a “WHERE KEY=HEX(00)” and got a low cost unique index lookup instead of a costly full table scan.
A second item of note, one of the tables in a SQL involved in a match against values passed into a global table temporary table had a PARALLEL setting of 4, but the instance had a parallel min servers setting of 0 so there was no way that a parallel scan could be performed. However, the fun part in this situation comes up when you get an explain plan. Using the command line explain plan you get a perfectly reasonable plan using an index scan on the permanent table and a full scan on the global temp. However, when you check on this SQL in the V$SQL_PLAN table it is attempted a parallel query with a single query slave doing a full table scan on both the permanent and temporary table!
The proper plan for this non-parallel query being forced into a non-optimal parallel plan was also shown in the EM database control session SQL screen (since it is probably pulled from the DBA_HIST_SQL_PLAN which is a child of the V$SQL_PLAN table, this makes sense.) However, the developer, using the PL/SQL Developer tool, was getting the bad explain plan and couldn’t see the problem. We changed the PARALLEL to 1 on the table and the plans then matched between the online explain plan and the V$SQL_PLAN explain plan.
A second note for Oracle: If the PARALLEL is set on a table and the MIN_PARALLEL_SERVERS is set to 0, completely disregard the PARALLEL setting. In testing a setting of 0 resulted in no P000 processes starting so even with the resulting parallel plan the query was serialized.
And a final note: The plan generated by the EXPLAIN PLAN and AUTOTRACE commands should match the one generated into the V$SQL_PLAN table.