Join Elimination

A question has just appeared on OTN describing a problem where code that works in 11g doesn’t work in 12c (exact versions not specified). The code in question is a C-based wrapper for some SQL, and the problem is a buffer overflow problem. The query supplied is as follows:

select T1.C1 from T1, T2 where T1.C1 = T2.D1;

The problem is that this works in 11g where the receiving (C) variable is declared as

char myBuffer [31];

but it doesn’t work in 12c unless the receiving variable is declared as:

char myBuffer [51];

There’s an important bit of background information that might be giving us a clue about what’s happened (although what I’m about to describe isn’t actually the problem unless the SQL provided is a simplified version of the problem SQL that is expected to display the problem). Column C1 is defined as char(30) and column D1 is defined as char(50). Here’s some sample code showing why you might need a buffer of 50+1 bytes to hold something that ought to be 30+1 bytes long. (This may be nothing to do with the anomaly described in the original posting – it’s just something I thought of when I first saw the question.)

rem     Script:         join_elimination_oddity.sql
rem     Author:         Jonathan Lewis
rem     Dated:          Mar 2017

create table t1(
        c30     char(30) primary key
);

create table t2(
        d50     char(50) references t1
);

begin
        dbms_stats.gather_table_stats(
                ownname          => user,
                tabname          =>'T1',
                method_opt       => 'for all columns size 1'
        );

        dbms_stats.gather_table_stats(
                ownname          => user,
                tabname          =>'T2',
                method_opt       => 'for all columns size 1'
        );
end;
/

explain plan for
select
        t1.c30
from
        t1, t2
where
        t1.c30 = t2.d50
;

select * from table(dbms_xplan.display(null,null,'projection'));

So we’re selecting c30 – the 30 byte character column – from t1; what do we actually get ? Here’s the plan. including the Column Projection Information:

--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |     1 |    52 |     1   (0)| 00:00:01 |
|*  1 |  TABLE ACCESS FULL| T2   |     1 |    52 |     1   (0)| 00:00:01 |
--------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------
   1 - filter("T2"."D50" IS NOT NULL)

Column Projection Information (identified by operation id):
-----------------------------------------------------------
   1 - "T2"."D50"[CHARACTER,50]

Table t1 has been eliminated and the projected column is the “equivalent” column from t2 – which is too long for the expected output. To work around this problem you can disable join elimination either by parameter (_optimizer_join_elimination_enabled=false) or by hinting /*+ no_eliminate_join(t1) */ in which case the plan (with my data) became a nested loop join from t2 to t1 with column c1 projected as expected.

Footnote:

Two things to note about my demonstration

• If you’re going to create a referential integrity constraint between columns they do need to be of exactly the same type. (If you want to get technical you might say “in the same domain” – but Oracle doesn’t do domains.)
• This extremely simple case demonstrates the problem in 11.2.0.4 as well as 12.1.0.2. Possibly a more complex query could be produced where (thanks to limitations in query transformations) 11g doesn’t spot the option for join elimination while 12c does; alternatively, a very simple two-column example in 11g won’t do join elimination while a two-column example in 12.2 can (though it doesn’t always) – so upgrading to 12.2 MIGHT cause more people to see this anomaly appearing.

Update (14/3/17)

A follow-up to the original posting has identified source of the problem. Rather than a side effect of join elimination the anomaly has appeared because of the “partial join evaluation” mechanism introduced in 12c. There is a hint to disable the mechanism /*+ no_partial_join(alias) */ or – if you can’t get hints into place for all the necessary code – it would be possible to disable the feature completely through the use of a hidden parameter “_optimizer_partial_join_eval”=false.  The usual rule of checking with Oracle support before setting hidden parameter applies, of course.

Update July 2021

The change in projection is still present in 19.11.0.0