Table Elimination in Other Databases

In addition to MariaDB, Table Elimination is found in both Microsoft SQL Server 2005/2008 and Oracle 11g. Of the two, Microsoft SQL Server 2005/2008 seems to have the most advanced implementation. Oracle 11g has been confirmed to use table elimination but not to the same extent.

To compare the two, we will look at the following query:

select
 A.colA
from
 tableA A
left outer join
 tableB B
on
 B.id = A.id;

When using A as the left table we ensure that the query will return at least as many rows as there are in that table. For rows where the join condition (B.id = A.id) is not met the selected column (A.colA) will still contain its original value. The not seen B.* row would contain all NULL:s.

However, the result set could actually contain more rows than what is found in tableA if there are duplicates of the column B.id in tableB. If A contains a row [1, "val1"] and B the rows [1, "other1a"],[1, "other1b"] then two rows will match in the join condition. The only way to know what the result will look like is to actually touch both tables during execution.

Instead, let's say tableB contains rows that make it possible to place a unique constraint on the column B.id, for example, which is often the case with a primary key. In this situation we know that we will get exactly as many rows as there are in tableA, since joining with tableB cannot introduce any duplicates. Furthermore, as in the example query, if we do not select any columns from tableB, touching that table during execution is unnecessary. We can remove the whole join operation from the execution plan.

Both SQL Server 2005/2008 and Oracle 11g deploy table elimination in the case described above. Let us look at a more advanced query, where Oracle fails.

select
 A.colA
from
 tableA A
left outer join
 tableB B
on
 B.id = A.id
and
 B.fromDate = (
   select
     max(sub.fromDate)
   from
     tableB sub
   where
     sub.id = A.id
 );

In this example we have added another join condition, which ensures that we only pick the matching row from tableB having the latest fromDate. In this case tableB will contain duplicates of the column B.id, so in order to ensure uniqueness the primary key has to contain the fromDate column as well. In other words the primary key of tableB is (B.id, B.fromDate).

Furthermore, since the subselect ensures that we only pick the latest B.fromDate for a given B.id we know that at most one row will match the join condition. We will again have the situation where joining with tableB cannot affect the number of rows in the result set. Since we do not select any columns from tableB, the whole join operation can be eliminated from the execution plan.

SQL Server 2005/2008 will deploy table elimination in this situation as well. We have not found a way to make Oracle 11g use it for this type of query. Queries like these arise in two situations. Either when you have a denormalized model consisting of a fact table with several related dimension tables, or when you have a highly normalized model where each attribute is stored in its own table. The example with the subselect is common whenever you store historized/versioned data.

See Also

• This page is based on the following blog post about table elimination: http://s.petrunia.net/blog/?p=58