5.11.5. Partitioning and Constraint Exclusion

5.11.5.分区和约束排除

Constraint exclusion is a query optimization technique similar to partition pruning. While it is primarily used for partitioning implemented using the legacy inheritance method, it can be used for other purposes, including with declarative partitioning.

约束排除是一个类似于分区排除的查询优化技术。虽然它主要用于使用继承方法实现的分区，但它也可以用于其他目的，包括原生分区。

Constraint exclusion works in a very similar way to partition pruning, except that it uses each table's CHECK constraints — which gives it its name — whereas partition pruning uses the table's partition bounds, which exist only in the case of declarative partitioning. Another difference is that constraint exclusion is only applied at plan time; there is no attempt to remove partitions at execution time.

约束排除的工作方式与分区排除类似，除了约束排除使用表的检查约束（为其命名），而分区排除使用仅原生分区支持的表分区边界。另一个不同点在于，约束排除仅在生成执行计划阶段有用，而在执行阶段无效。

The fact that constraint exclusion uses CHECK constraints, which makes it slow compared to partition pruning, can sometimes be used as an advantage: because constraints can be defined even on declaratively-partitioned tables, in addition to their internal partition bounds, constraint exclusion may be able to elide additional partitions from the query plan.

虽然约束排除使用了检查约束，这使得其与分区排除比起来慢很多，但有时候，这可以变为优势：因为甚至可以在原生分区的表上定义约束，所以除了其内部分区边界外，约束排除还可以帮助从查询计划中排除其他更多分区。

The default (and recommended) setting of constraint_exclusion is neither on nor off, but an intermediate setting called partition, which causes the technique to be applied only to queries that are likely to be working on inheritance partitioned tables. The on setting causes the planner to examine CHECK constraints in all queries, even simple ones that are unlikely to benefit.

constraint_exclusion的默认值（也是推荐值）既不是on也不是off，而是partition，这使得该技术仅应用于可能在继承分区表上运行的查询。 on设置使执行计划时检查所有查询中的CHECK约束，即使是那些不太可能受益的简单查询也会如此。

The following caveats apply to constraint exclusion:

约束排除有关注意事项如下：

•约束排除仅在执行计划期间有效；与分区排除不同，约束排除在执行期间无效。

• Constraint exclusion only works when the query's WHERE clause contains constants (or externally supplied parameters). For example, a comparison against a non-immutable function such as CURRENT_TIMESTAMP cannot be optimized, since the planner cannot know which child table the function's value might fall into at run time.

• 约束排除仅在查询的WHERE子句包含常量（或外部提供的参数）时有效。例如，无法优化与诸如CURRENT_TIMESTAMP之类的不可修改函数的比较，因为执行计划无法在运行时知道该函数的值可能属于哪个子表。

• Keep the partitioning constraints simple, else the planner may not be able to prove that child tables might not need to be visited. Use simple equality conditions for list partitioning, or simple range tests for range partitioning, as illustrated in the preceding examples. A good rule of thumb is that partitioning constraints should contain only comparisons of the partitioning column(s) to constants using B-tree-indexable operators, because only B-tree-indexable column(s) are allowed in the partition key.

•分区约束要简单，否则的话执行计划可能无法明确哪个子表不需要访问。如上例所示，对列表分区使用简单的相等条件，对范围分区使用简单的范围测试。一个好的经验法则是，分区约束应只包含使用B-tree-indexable运算符的分区列与常量的比较，因为在分区键中仅允许B-tree-indexable列。

• All constraints on all children of the parent table are examined during constraint exclusion, so large numbers of children are likely to increase query planning time considerably. So the legacy inheritance based partitioning will work well with up to perhaps a hundred child tables; don't try to use many thousands of children.

•约束排除会检查父表的所有子表中的所有约束，所以大量的子表会显著提高查询时间。因此，基于继承的分区最多也就有几百个子表；千万不要尝试使用几千个子表。

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