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From "Jeff Lichtman (JIRA)" <derby-...@db.apache.org>
Subject [jira] Created: (DERBY-634) Subquery materialization can cause stack overflow
Date Thu, 20 Oct 2005 20:17:53 GMT
Subquery materialization can cause stack overflow

         Key: DERBY-634
         URL: http://issues.apache.org/jira/browse/DERBY-634
     Project: Derby
        Type: Bug
  Components: SQL  
    Reporter: Jeff Lichtman

A performance optimization in subquery processing can cause a stack overflow.

The optimization materializes a subquery ResultSet in memory where it thinks the rows will
fit in memory. The materialization is done as a  set of  nested unions of constant rows (UnionResultSets
and RowResultSets). If there are a lot of rows this can cause a stack overflow when fetching
a row.

The obvious fix is to make it use an iterative technique rather than a recursive one for storing
and returning the rows. See the method BaseActivation.materializeResultSetIfPossible() in
the language execution code.

There are some other issues with this performance optimization that should be looked at:

1) The optimization can backfire, making the query run much slower. For example, in the query:

    select * from one_row_table where column1 not in
        (select column2 from million_row_table)

reading million_row_table into memory is an expensive operation. If there is an index on million_row_table.column2,
the query should return a result very quickly despite the large size of million_row_table
by doing a single probe into million_row_table via the index.

Since in-memory materialization can be an expensive operation, the decision about whether
to do it should be made based on query optimizer cost estimates. See SubqueryNode.generateExpression().

2) It may not be wise to cache partial query results in memory at all. Although this can help
performance in some cases, it also chews up memory. This is different from a limited-size
cache with a backing store (like what the store uses for page caching). The language has no
way to limit the total amount of memory used in this type of processing. Note that hash joins
originally used in-memory hash tables with no backing store, and that a backing store was
added later.

3) The implementation of this optimization has some problems. The decision to materialize
the subquery results in memory is made during code generation - all such decisions should
be made during the compilation phase. It's not clear to me why materializeResultSetIfPossible()
is in BaseActivation - I would expect the of materialization to be done by a type of ResultSet,
not by a method in BaseActivation. Also, this method calls getMaxMemoryPerTable() in the OptimizerFactory
- nothing in the execution code should refer to anything in the compilation code (perhaps
getMaxMemoryPerTable() should be moved somewhere else).

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