Question

We’re using a PL/SQL table (named pTable) to collect a number of ids to be updated.

However, the statement

UPDATE aTable
SET aColumn = 1
WHERE id IN (SELECT COLUMN_VALUE
                     FROM   TABLE (pTable));

takes a long time to execute.

It seems that the optimizer comes up with a very bad execution plan, instead of using the index that is defined on id (as the primary key) it decides to use a full table scan on the aTable. pTable usually contains very few values (in most cases just one).

What can we do to make this faster? The best we’ve come up with is to handle low pTable.Count (1 and 2) as special cases, but that is certainly not very elegant.

Thanks for all the great suggestions. I wrote about this issue in my blog at http://smartercoding.blogspot.com/2010/01/performance-issues-using-plsql-tables.html.

Answer 1

You can try the cardinality hint. This is good if you know (roughly) the number of rows in the collection.

UPDATE aTable 
SET aColumn = 1 
WHERE id IN (SELECT /*+ cardinality( pt 10 ) */ 
                     COLUMN_VALUE 
              FROM   TABLE (pTable) pt );