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When automatic reoptimization plan is less efficient

11gR2 started to have the optimizer react at execution time when a misestimate is encountered. Then the next executions are re-optimized with more accurate estimation, derived from the execution statistics. This was called cardinality feedback. Unfortunately, in rare cases we had a fast execution plan with bad estimations, and better estimations lead to worse execution plan. This is rare, but even when 9999 queries are faster, the one that takes too long will gives a bad perception of this optimizer feature.

An Organizational Constraint that Diminishes Software Quality

One of the biggest problems in software performance today occurs when the people who write software are different from the people who are required to solve the performance problems that their software causes. It works like this:

  1. Architects design a system and pass the specification off to the developers.
  2. The developers implement the specs the architects gave them, while the architects move on to design another system.
  3. When the developers are “done” with their phase, they pass the code off to the production operations team. The operators run the system the developers gave them, while the developers move on to write another system.

The process is an assembly line for software: architects specialize in architecture, developers specialize in development, and operators specialize in operating. It sounds like the principle of industrial efficiency taken to its logical conclusion in the software world.