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Sergey Petrunya 2012-08-21 22:24:34 +04:00
parent fbee9f5b6e
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sql/opt_subselect.cc
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@ -143,18 +143,18 @@
3.2.1 Non-merged semi-joins and join optimization 3.2.1 Non-merged semi-joins and join optimization
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
For join optimization purposes, non-merged semi-join nests are similar to For join optimization purposes, non-merged semi-join nests are similar to
base tables - they've got one JOIN_TAB, which can be accessed with one of base tables. Each such nest is represented by one one JOIN_TAB, which has
two methods: two possible access strategies:
- full table scan (representing SJ-Materialization-Scan strategy) - full table scan (representing SJ-Materialization-Scan strategy)
- eq_ref-like table lookup (representing SJ-Materialization-Lookup) - eq_ref-like table lookup (representing SJ-Materialization-Lookup)
Unlike regular base tables, non-merged semi-joins have: Unlike regular base tables, non-merged semi-joins have:
- non-zero JOIN_TAB::startup_cost, and - non-zero JOIN_TAB::startup_cost, and
- join_tab->table->is_filled_at_execution()==TRUE, which means one - join_tab->table->is_filled_at_execution()==TRUE, which means one
cannot do const table detection or range analysis or other table data- cannot do const table detection, range analysis or other dataset-dependent
dependent inferences optimizations.
// instead, get_delayed_table_estimates() runs optimization on the nest so that Instead, get_delayed_table_estimates() will run optimization for the
// we get an idea about temptable size subquery and produce an E(materialized table size).
3.2.2 Merged semi-joins and join optimization 3.2.2 Merged semi-joins and join optimization
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~