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是否可以在 PostgreSQL 中创建唯一索引或其他类型的约束，以便在事务提交时检查而不是提前一毫秒？
我需要一对 (record_id, ordering) 的索引，所以我确保在给定的 record_id 中只有一个且不超过一个记录具有相同的顺序。问题在哪里？好吧，问题在于我正在使用的网络框架处理重新排序项目的方式。看起来，当一个项目被移动时，当它的排序顺序发生变化时，框架会用新的排序值写入新项目，然后不久之后它会更新另一个项目，从而造成一种临时情况，其中不止一条记录具有相同的订购值(value)。重新排序所有内容后，所有记录都会更新，并且在事务 COMMIT 上一切都应该再次正常。
如果重要的话，我正在使用 PostgreSQL 10。
最佳答案
使用SET CONSTRAINTS命令:
SET CONSTRAINTS
SET CONSTRAINTS — set constraint check timing for the current transaction
Synopsis
SET CONSTRAINTS { ALL | name [, ...] } { DEFERRED | IMMEDIATE }Description
SET CONSTRAINTS sets the behavior of constraint checking within the current transaction. IMMEDIATE constraints are checked at the end of each statement. DEFERRED constraints are not checked until transaction commit. Each constraint has its own IMMEDIATE or DEFERRED mode.
Upon creation, a constraint is given one of three characteristics: DEFERRABLE INITIALLY DEFERRED, DEFERRABLE INITIALLY IMMEDIATE, or NOT DEFERRABLE. The third class is always IMMEDIATE and is not affected by the SET CONSTRAINTS command. The first two classes start every transaction in the indicated mode, but their behavior can be changed within a transaction by SET CONSTRAINTS.
SET CONSTRAINTS with a list of constraint names changes the mode of just those constraints (which must all be deferrable). Each constraint name can be schema-qualified. The current schema search path is used to find the first matching name if no schema name is specified. SET CONSTRAINTS ALL changes the mode of all deferrable constraints.
When SET CONSTRAINTS changes the mode of a constraint from DEFERRED to IMMEDIATE, the new mode takes effect retroactively: any outstanding data modifications that would have been checked at the end of the transaction are instead checked during the execution of the SET CONSTRAINTS command. If any such constraint is violated, the SET CONSTRAINTS fails (and does not change the constraint mode). Thus, SET CONSTRAINTS can be used to force checking of constraints to occur at a specific point in a transaction.
Currently, only UNIQUE, PRIMARY KEY, REFERENCES (foreign key), and EXCLUDE constraints are affected by this setting. NOT NULL and CHECK constraints are always checked immediately when a row is inserted or modified (not at the end of the statement). Uniqueness and exclusion constraints that have not been declared DEFERRABLE are also checked immediately.
The firing of triggers that are declared as “constraint triggers” is also controlled by this setting — they fire at the same time that the associated constraint should be checked.
您还可以在(例如)CREATE TABLE 的引用文档中找到它
[ CONSTRAINT constraint_name ]
{ NOT NULL |
NULL |
CHECK ( expression ) [ NO INHERIT ] |
DEFAULT default_expr |
GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY [ ( sequence_options ) ] |
UNIQUE index_parameters |
PRIMARY KEY index_parameters |
REFERENCES reftable [ ( refcolumn ) ] [ MATCH FULL | MATCH PARTIAL | MATCH SIMPLE ]
[ ON DELETE action ] [ ON UPDATE action ] }
[ DEFERRABLE | NOT DEFERRABLE ] [ INITIALLY DEFERRED | INITIALLY IMMEDIATE ]
约束可以是:
关于sql - PostgreSQL 约束，在提交时检查，而不是更早，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/50098797/