2000-01-28：在WHERE子句中适当地使用DNF而不是CNF
25 年前的今天（PostgreSQL 7.0 开发周期），进行了一项更改，使得在适当的情况下可以使用 DNF（“析取范式”）而不是 CNF（“合取范式”）。
commit 003dd965d2380823624f58d0d0cf72c4edc3bcb4 Author: Tom Lane <tgl@sss.pgh.pa.us> Date: Fri Jan 28 03:22:36 2000 +0000 Apply the heuristic proposed by Taral (see pgsql-general archives for 2-Oct-98 or TODO.detail/cnfify) to decide whether we want to reduce WHERE clause to CNF form, DNF form, or neither. This is a HUGE win. The heuristic conditions could probably still use a little tweaking to make sure we don't pick CNF when DNF would be better, or vice versa, but the risk of exponential explosion in cnfify() is gone. I was able to run ten-thousand-AND-subclause queries through the planner in a reasonable amount of time.
从实际角度来看，这意味着可以更有效地处理更多的 AND / OR 子句，并避免了在 DNF 更有效的情况下使用 CNF 可能导致的内存不足错误。
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