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Administration Guide

Overview of the SQL Compiler

The SQL compiler performs several steps before producing an access plan that you can execute. These steps are shown in Figure 43.

Figure 43. Steps performed by SQL Compiler


This diagram shows that the Query Graph Model is a key component of the SQL compiler. The query graph model is an internal, in-memory database that is used to represent the query throughout the query compilation process as described below:

It is recommended that RUNSTATS be done periodically on tables used in queries where good performance is desired. The optimizer will then be better equipped with relevant statistical information on the nature of the data. If RUNSTATS is not done (or the optimizer suspects that RUNSTATS was done on empty or near empty tables), the optimizer may either use defaults or attempt to derive certain statistics based on the number of file pages used to store the table on disk (FPAGES).

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