# Overriding the default optimizer behavior

You can override the default behavior of the Derby query optimizer by including a --DERBY-PROPERTIES clause and an associated property as a comment within an SQL statement.

Because optimizer overrides are expressed as comments, they must be included at the end of a line. You can specify optimizer override properties for an entire FROM clause, for tables in the FROM clause, or for both.

The syntax for a FROM clause property is:

    FROM [ -- DERBY-PROPERTIES joinOrder = { FIXED | UNFIXED } ]
         TableExpression [,TableExpression]*

The syntax for table optimizer override properties, which must be included at the end of a TableExpression, is:

    {table-Name | view-Name }
         [ [ AS ] correlation-Name
         [ (Simple-column-Name [ , Simple-column-Name ]* ) ] ]
         [ -- DERBY-PROPERTIES { constraint = constraint-Name | index = index-Name | joinStrategy = { NESTEDLOOP | HASH } } ]

The space between -- and DERBY-PROPERTIES is optional.

Important: Make sure that you adhere to the correct syntax when using the --DERBY-PROPERTIES clause. Failure to do so can cause the parser to interpret it as a comment and ignore it. To verify that the parser interpreted your overrides correctly, you can use RunTimeStatistics. See Optimizer overrides for more information.
The following four properties are available for use in a --DERBY-PROPERTIES clause:

constraint
  To force the use of the index that enforces a primary key, a foreign key, or unique constraint, use the constraint property and specify the unqualified name of the constraint. The constraint property can be used only within a TableExpression, and it can be specified only on base tables; it cannot be specified on views or derived tables.
index
  The index property is similar to the constraint property. To force use of a particular index, specify the unqualified index name. To force a table scan, specify null for the index name. The index property can be used only within a TableExpression, and it can be specified only on base tables; it cannot be specified on views or derived tables.
joinOrder
  Use the joinOrder property to override the optimizer’s choice of join order for two tables. When the value FIXED is specified, the optimizer will choose the order of tables as they appear in the FROM clause as the join order. Valid values for the joinOrder property include FIXED and UNFIXED. The joinOrder property can be used with a FROM clause.
joinStrategy
  Use the joinStrategy property to override the optimizer’s choice of join strategy. The two types of join strategy are called nested loop and hash. In a nested loop join strategy, for each qualifying row in the outer table, Derby uses the appropriate access path (index or table scan) to find the matching rows in the inner table. In a hash join strategy, Derby constructs a hash table that represents the inner table. For each qualifying row in the outer table, Derby does a quick lookup on the hash table to find the matching rows in the inner table. Derby needs to scan the inner table or index only once to create the hash table. The --DERBY-PROPERTIES parameter must immediately follow the inner table.

  Typically, you will use the joinStrategy property only in conjunction with the joinOrder property. Specifying a join strategy without knowing the join order can result in less-than-optimal performance.

  Valid values include HASH and NESTEDLOOP. The joinStrategy property can be used only within a TableExpression.

The following examples illustrate the use of the --DERBY-PROPERTIES clause:

constraint

      CREATE TABLE t1 (c1 int, c2 int, c3 int, CONSTRAINT cons1 PRIMARY KEY (c1, c2))
      INSERT INTO t1 VALUES (1, 1, 1), (2, 2, 2), (3, 3, 3), (4, 4, 4)
      SELECT * FROM t1 --DERBY-PROPERTIES constraint=cons1
      FOR UPDATE

index

      CREATE TABLE t1 (c1 int, c2 int, c3 int, CONSTRAINT cons1 PRIMARY KEY (c1, c2))
      INSERT INTO t1 VALUES (1, 1, 1), (2, 2, 2), (3, 3, 3), (4, 4, 4)
      CREATE INDEX t1_c1 ON t1(c1)
      SELECT * FROM t1 --DERBY-PROPERTIES index=t1_c1
      WHERE c1=1

joinOrder

      CREATE TABLE t1 (c1 int, c2 int, c3 int, CONSTRAINT cons1 PRIMARY KEY (c1, c2))
      CREATE TABLE t2 (c1 int not null, c2 int not null, c3 int, CONSTRAINT cons2 UNIQUE (c1, c2))
      INSERT INTO t1 VALUES (1, 1, 1), (2, 2, 2), (3, 3, 3), (4, 4, 4)
      INSERT INTO t2 VALUES (1, 1, 1), (2, 2, 2), (3, 3, 3), (4, 4, 4)
      SELECT * FROM --DERBY-PROPERTIES joinOrder=FIXED
      t1, t2
      WHERE t1.c1=t2.c1

joinStrategy

      CREATE TABLE t1 (c1 int, c2 int, c3 int, CONSTRAINT cons1 PRIMARY KEY (c1, c2))
      CREATE TABLE t2 (c1 int not null, c2 int not null, c3 int, CONSTRAINT cons2 UNIQUE (c1, c2))
      INSERT INTO t1 VALUES (1, 1, 1), (2, 2, 2), (3, 3, 3), (4, 4, 4)
      INSERT INTO t2 VALUES (1, 1, 1), (2, 2, 2), (3, 3, 3), (4, 4, 4)
      SELECT * FROM --DERBY-PROPERTIES joinOrder=FIXED
      t1 a, t1 b --DERBY-PROPERTIES joinStrategy=NESTEDLOOP
      WHERE a.c1=b.c1

Parent topic: DML statements and performance
Related concepts
Performance and optimization
Locking and performance
Non-cost-based optimizations