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SQL Anywhere 11.0.0 » SQL Anywhere Server - SQL Usage » Query Optimizer » Query optimization and execution


Phases of query processing

Optimization is essential in generating a suitable access plan for a query. Once each query is parsed, the optimizer analyzes it and decides on an access plan that computes the result using as few resources as possible. Optimization begins just before execution. If you are using cursors in your application, optimization commences when the cursor is opened. Unlike many other commercial database systems, SQL Anywhere usually optimizes each statement just before executing it. Because SQL Anywhere performs just-in-time optimization of each statement, the optimizer has access to the values of host and stored procedure variables, which allows for better selectivity estimation analysis. In addition, just-in-time optimization allows the optimizer to adjust its choices based on the statistics saved after previous query executions.

The following list describes the phases that a statement goes through starting with the annotation phase and ending with its execution. It also describes the assumptions that underlie the design of the optimizer, and discusses selectivity estimation, cost estimation, and the other steps of optimization.

  • Annotation phase   When the database server receives a query, it uses a parser to parse the statement and transform it into an algebraic representation of the query, also known as a parse tree. At this stage the parse tree is used for semantic and syntactic checking (for example, validating that objects referenced in the query exist in the catalog), permission checking, KEY JOINs and NATURAL JOINs transformation using defined referential constraints, and non-materialized view expansion. The output of this phase is a rewritten query, in the form of a parse tree, which contains annotation to all of the objects referenced in the original query.

    When referencing a temporary table from multiple procedures, a potential issue can arise if the temporary table definitions are inconsistent and statements referencing the table are cached. See Referencing temporary tables within procedures.

  • Query rewrite phase   During this phase, the query undergoes iterative semantic transformations. While the query is still represented as an annotated parse tree, rewrite optimizations, such as join elimination, DISTINCT elimination, and predicate rewriting, are applied in this phase. The semantic transformations in this phase are performed based on semantic transformation rules that are applied heuristically to the parse tree representation.

  • Optimization phase   The optimization phase uses a different internal representation of the query, the query optimization structure, which is built from the parse tree.

    • Pre-optimization phase   The pre-optimization phase completes the optimization structure with the information needed later in the enumeration phase. During this phase the query is analyzed in order to find all relevant indexes and materialized views that may be used in the query access plan. For example, in this phase, the View Matching algorithm determines all the materialized views that may be used to satisfy part or all of the query. In addition, based on query predicate analysis, the optimizer builds alternative join methods that may be used in the enumeration phase to join the query's tables. During this phase, no decision is made regarding the best access plan for the query; the goal of this phase is to prepare for the enumeration phase.

    • Enumeration phase   During this phase, the optimizer enumerates possible access plans for the query using the building blocks generated in the pre-optimization phase. The search space is very large and the optimizer uses a proprietary enumeration algorithm to generate and prune the generated access plans. For each plan, cost estimation is computed, which is used to compare the current plan with the best plan found so far. Expensive plans are discarded during these comparisons. Cost estimation takes into account resource utilization such as disk and CPU operations, the estimated number of rows of the intermediate results, optimization goal, cache size, and so on. The output of the enumeration phase is the best access plan for the query.

  • Plan building phase   The plan building phase takes the best access plan and builds the corresponding final representation of the query execution plan used to execute the query. You can see a graphical version of the plan in the Plan Viewer in Interactive SQL. The graphical plan has a tree structure where each node is a physical operator implementing a specific relational algebraic operation, for example, Hash Join and Ordered Group By are physical operators implementing a join and a group by operation, respectively. See Reading graphical plans.

  • Execution phase   The result of the query is computed using the query execution plan built in the previous phase.

Statements that have no result sets, such as UPDATE or DELETE statements, also have a query execution plan.

Queries that bypass optimization

Almost all statements pass through the phases described above, with two exceptions: simple queries that bypass the optimizer, and queries whose plans are already cached by the database server.

If a query is recognized as a simple query, a heuristic rather than cost-based optimization is used—that is, the optimization phase is skipped and the query execution plan is built directly from the parse tree representation of the query. A simple query is a DYNAMIC SCROLL or NO SCROLL cursor that does not contain any kind of subquery, more than one table, a proxy table, a user-defined function, NUMBER(*), UNION, aggregation, DISTINCT, GROUP BY, or more than one predicate on a single column. Simple queries can contain ORDER BY only as long as the WHERE clause contains conditions on each of the primary key columns.

For queries contained inside stored procedures and user-defined functions, the database server may cache the execution plans so that they can be reused. For this class of queries, the query execution plan is cached after execution. The next time the query is executed, the plan is retrieved and all the phases up to the execution phase are skipped. See Plan caching.

You can force the database server to optimize a simple query, or to optimize a stored procedure or user-defined functions that contains a query, by using the OPTION FORCE OPTIMIZATION clause in the SELECT statement. When this clause is specified, plan caching is not used. See SELECT statement.