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SAP Sybase SQL Anywhere 16.0 » SQL Anywhere Server - SQL Usage » Queries and data modification » Queries » Advanced: Query optimization » How the optimizer works


Subquery and function caching

When SQL Anywhere processes a subquery, it caches the result. This caching is done on a request-by-request basis; cached results are never shared by concurrent requests or connections. Should SQL Anywhere need to re-evaluate the subquery for the same set of correlation values, it can simply retrieve the result from the cache. In this way, SQL Anywhere avoids many repetitious and redundant computations. When the request is completed (the query's cursor is closed), SQL Anywhere releases the cached values.

As the processing of a query progresses, SQL Anywhere monitors the frequency with which cached subquery values are reused. If the values of the correlated variable rarely repeat, then SQL Anywhere needs to compute most values only once. In this situation, SQL Anywhere recognizes that it is more efficient to recompute occasional duplicate values, than to cache numerous entries that occur only once. So, the database server suspends the caching of this subquery for the remainder of the statement and proceeds to re-evaluate the subquery for each and every row in the outer query block.

SQL Anywhere also does not cache if the size of the dependent column is more than 255 bytes. In such cases, you may want to rewrite your query or add another column to your table to make such operations more efficient.

 Function caching
 See also