13.4. Subquery Optimization
13.4. Subquery Optimization EXISTS subqueries are typically rewrite to "SELECT 1 FROM ..." to prevent unnecessary evaluation of SELECT expressions. Quantified compare SOME subqueries are always turned into an equivalent IN predicate or comparison against an aggregate value. e.g. col > SOME (select col1 from table) would become col > (select min(col1) from table) Uncorrelated EXISTs and scalar subquery that are not pushed to the source can be evaluated prior to source command formation. Correlated subqueries used in DELETEs or UPDATEs that are not pushed as part of the corresponding DELETE/UPDATE will cause JBoss Data Virtualization to perform row-by-row compensating processing. This will only happen if the affected table has a primary key. If it does not, then an exception will be thrown. WHERE or HAVING clause IN, Quantified Comparison, Scalar Subquery Compare, and EXISTs predicates can take the MJ (merge join), DJ (dependent join), or NO_UNNEST (no unnest) hints appearing just before the subquery. The MJ hint directs the optimizer to use a traditional, semijoin, or antisemijoin merge join if possible. The DJ is the same as the MJ hint, but additionally directs the optimizer to use the subquery as the independent side of a dependent join if possible. The NO_UNNEST hint, which supercedes the other hints, will direct the optimizer to leave the subquery in place. Example 13.2. Merge Join Hint Usage SELECT col1 from tbl where col2 IN /*+ MJ */ (SELECT col1 FROM tbl2) Example 13.3. Dependent Join Hint Usage SELECT col1 from tbl where col2 IN /*+ DJ */ (SELECT col1 FROM tbl2) Example 13.4. No Unnest Hint Usage SELECT col1 from tbl where col2 IN /*+ NO_UNNEST */ (SELECT col1 FROM tbl2) The system property org.teiid.subqueryUnnestDefault controls whether the optimizer will by default unnest subqueries. If true, then most non-negated WHERE or HAVING clause non-negated EXISTS or IN subquery predicates can be converted to a traditional join. The planner will always convert to anitjoin or semijoin vartiants is costing is favorable. Use a hint to override this behavior if needed. EXISTs and scalar subqueries that are not pushed down, and not converted to merge joins, are implicitly limited to 1 and 2 result rows respectively. Conversion of subquery predicates to nested loop joins is not yet available.
SELECT col1 from tbl where col2 IN /*+ MJ */ (SELECT col1 FROM tbl2) SELECT col1 from tbl where col2 IN /*+ DJ */ (SELECT col1 FROM tbl2) SELECT col1 from tbl where col2 IN /*+ NO_UNNEST */ (SELECT col1 FROM tbl2)