Question

is there a difference in the Informix query performance using ANSI syntax:

SELECT .. 
  LEFT OUTER JOIN some_table ON (<condition>) 
  LEFT OUTER JOIN some_other_table (<condition_on_some_table>)

and Informix specific OUTER syntax:

SELECT ... 
  OUTER (some_table, 
   OUTER(some_other_table)) 
WHERE <join_conditions>

thanks

Answer 1

Yes, there are differences in the semantics of a Standard outer join and an Informix-style outer join which inevitably means that there are differences in the query plan.

In general, use the Standard notation for any new or modified code – leave the Informix-style outer join notation for (unchanged) legacy code, and preferably update even that to use the new join notations.

What is the difference? Fair question – hard to explain, and harder still to come up with a good (plausible example). Basically, the Informix-style notation preserves the rows from the ‘dominant’ tables (the non-outer tables) even when there are criteria based on the values in the outer-joined table that would reject those rows.

These two queries produce the same result:

SELECT i.*, o.*
  FROM DominantTable AS i, OUTER OuterJoinedTable AS o
 WHERE i.pk_column = o.fk_column;

SELECT i.*, o.*
  FROM DominantTable AS i
  LEFT OUTER JOIN OuterJoinedTable AS o
    ON i.pk_column = o.fk_column;

These two queries do not necessarily produce the same result:

SELECT i.*, o.*
  FROM DominantTable AS i, OUTER OuterJoinedTable AS o
 WHERE i.pk_column = o.fk_column
   AND (o.alt_column IS NULL OR o.alt_column = 1);

SELECT i.*, o.*
  FROM DominantTable AS i
  LEFT OUTER JOIN OuterJoinedTable AS o
    ON i.pk_column = o.fk_column
 WHERE (o.alt_column IS NULL OR o.alt_column = 1);

The difference occurs in a situation like this:

DominantTable                              OuterJoinedTable
pk_column   other_column                   fk_column   alt_column
1           twenty                         1           3

The standard LEFT OUTER JOIN notation will produce the empty set as the result. The Informix-style join will produce the result:

pk_column   other_column   fk_column   alt_column
1           twenty         null        null

The data from DominantTable was not rejected because of a filter condition on the dominant table, so it is preserved by Informix. The standard join does the outer join and then filters the result.