Oracle Wait Interface: A Practical Guide to Performance Diagnostics & Tuning

Overhead

Performance sampling with SQL has some overheads in the sense that the SQL statements are subject to Oracle’s architecture and limits. The SQL statements must be parsed and optimized before execution. This involves dictionary lookups, latch acquisitions, etc. During the execution of the statements, the process must compete for resources as other Oracle processes. This involves latch and buffer lock acquisitions, consistent read image constructions, and various other potential waits. All these activities add to the overhead, and the overhead increases with higher sampling frequencies. But accessing the SGA directly from an external program does not introduce this kind of overhead to the Oracle instance. However, both types of access require ...