An access plan consists of a tree of relational algebra operators which, starting at the leaves of the tree, consume the base inputs to the query (usually rows from a table) and process the rows from bottom to top, so that the root of the tree yields the final result. Access plans can be viewed graphically for ease of comprehension. See Reading execution plans, and Reading graphical plans.
SQL Anywhere supports multiple implementations of these various relational algebra operations. For example, SQL Anywhere supports three different implementations of inner join: nested loops join, merge join, and hash join. Each of these operators can be advantageous to use in specific circumstances: some of the parameters that the query optimizer analyzes to make its choice include the amount of table data in cache, the characteristics and selectivity of the join predicate, the sortedness of the inputs to the join and the output from it, the amount of memory available to perform the join, and a variety of other factors.
SQL Anywhere may dynamically, at execution time, switch from the physical algebraic operator chosen by the optimizer to a different physical algorithm that is logically equivalent to the original. Typically, this alternative access plan is used in one of two circumstances:
When the total amount memory used to execute the statement is close to a memory governor threshold, then a switch is made to a strategy that may execute more slowly, but that frees a substantial amount of memory for use by other operators (or other requests). When this occurs, the QueryLowMemoryStrategy property is incremented. This information also appears in the graphical plan for the statement. For information about the QueryLowMemoryStrategy property, see Connection properties.
The amount of memory that can be used by an operator dependent upon the multiprogramming level of the server, and the number of active connections.
For more information about how the memory governor and the multiprogramming level, see:
If, at the beginning of its execution, the specific operator (a hash inner join, for example) determines that its inputs are not of the expected cardinality as that computed by the optimizer at optimization time. In this case, the operator may switch to a different strategy that will be less expensive to execute. Typically, this alternative strategy utilizes index nested loops processing. For the case of hash join, the QueryJHToJNLOptUsed property is incremented when this switch occurs. The occurrence of the join method switch is also included in the statement's graphical plan. For information about the QueryJHToJNLOptUsed property, see Connection properties.
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