MySQL Internals Manual  /  ...  /  Joins and Access Methods Joins and Access Methods

Bad join choices can cause more damage than bad choices in single-table searches, so MySQL developers have spent proportionally more time making sure that the tables in a query are joined in an optimal order and that optimal access methods (often called access paths) are chosen to retrieve table data. A combination of a fixed order in which tables are joined and the corresponding table access methods for each table is called query execution plan (QEP). The goal of the query optimizer is to find an optimal QEP among all such possible plans. There are several general ideas behind join optimization.

Each plan (or part of plan) is assigned a cost. The cost of a plan reflects roughly the resources needed to compute a query according to the plan, where the main factor is the number of rows that will be accessed while computing a query. Once we have a way to assign costs to different QEPs we have a way to compare them. Thus, the goal of the optimizer is to find a QEP with minimal cost among all possible plans.

In MySQL, the search for an optimal QEP is performed in a bottom-up manner. The optimizer first considers all plans for one table, then all plans for two tables, and so on, until it builds a complete optimal QEP. Query plans that consist of only some of the tables (and predicates) in a query are called partial plans. The optimizer relies on the fact that the more tables that are added to a partial plan, the greater its cost. This allows the optimizer to expand with more tables only the partial plans with lower cost than the current best complete plan.

The key routine that performs the search for an optimal QEP is sql/, find_best(). It performs an exhaustive search of all possible plans and thus guarantees it will find an optimal one.

Below we represent find_best() in an extremely free translation to pseudocode. It is recursive, so some input variables are labeled so far to indicate that they come from a previous iteration.

remaining_tables = {t1, ..., tn}; /* all tables referenced in a query */

procedure find_best(
   partial_plan in,      /* in, partial plan of tables-joined-so-far */
   partial_plan_cost,    /* in, cost of partial_plan */
   remaining_tables,     /* in, set of tables not referenced in partial_plan */
   best_plan_so_far,     /* in/out, best plan found so far */
   best_plan_so_far_cost)/* in/out, cost of best_plan_so_far */
   for each table T from remaining_tables
     /* Calculate the cost of using table T. Factors that the
        optimizer takes into account may include:
          Many rows in table (bad)
          Many key parts in common with tables so far (very good)
          Restriction mentioned in the WHERE clause (good)
          Long key (good)
          Unique or primary key (good)
          Full-text key (bad)
        Other factors that may at some time be worth considering:
          Many columns in key
          Short average/maximum key length
          Small table file
          Few levels in index
          All ORDER BY / GROUP columns come from this table */
     cost = complex-series-of-calculations;
     /* Add the cost to the cost so far. */
     partial_plan_cost+= cost;

     if (partial_plan_cost >= best_plan_so_far_cost)
       /* partial_plan_cost already too great, stop search */

     partial_plan= expand partial_plan by best_access_method;
     remaining_tables= remaining_tables - table T;
     if (remaining_tables is not an empty set)
       find_best(partial_plan, partial_plan_cost,
                 best_plan_so_far, best_plan_so_far_cost);
       best_plan_so_far_cost= partial_plan_cost;
       best_plan_so_far= partial_plan;

Here the optimizer applies a depth-first search algorithm. It performs estimates for every table in the FROM clause. It will stop a search early if the estimate becomes worse than the best estimate so far. The order of scanning will depend on the order that the tables appear in the FROM clause.

See: /sql/table.h, struct st_table.

ANALYZE TABLE may affect some of the factors that the optimizer considers.

See also: /sql/, make_join_statistics().

The straightforward use of find_best() and greedy_search() will not apply for LEFT JOIN or RIGHT JOIN. For example, starting with MySQL 4.0.14, the optimizer may change a left join to a straight join and swap the table order in some cases. See also LEFT JOIN and RIGHT JOIN Optimization.

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