如何为单个迁移设置 statement_timeout？

Question

我想为个人迁移设置 postgres statement_timeout。我似乎无法做到这一点。这是我的实验：

def change
  execute <<~SQL
    SET LOCAL statement_timeout = 1; -- ms

    -- this does not cause a timeout which is expected, because pg
    -- only applies the timeout to the next protocol message / statement,
    -- and rails sends everthing inside execute in the same statement
    select pg_sleep(1); -- seconds
  SQL

  # if uncommented, this DOES cause a timeout, which is expected
  # execute <<~SQL
  #   select pg_sleep(1); -- seconds
  # SQL

  # this does not cause a timeout, which is unexpected
  remove_column :foos, :bar

  # we do get here, which is unexpected
  raise "we finished"
end

我该怎么做？

Answer 1

您可以尝试使用较低级别 exec_query 或使用 execute_and_clear 甚至更低级别或使用 exec_no_cache 更低级别。

Answer 2

我假设你的问题是关于在你的迁移中为个人声明设置statement_timeout（而不是个人迁移 ).

您可以使用 Postgres’s SET instead of SET LOCAL 来实现。

class SomeMigration < ActiveRecord::Migration[6.0]
  # Disables BEGIN/COMMIT around migration. SET LOCAL will have no effect.
  disable_ddl_transaction!

  def change
    # Applies to the current database session, until changed or reset.
    execute <<~SQL
      SET statement_timeout = '1s';
    SQL

    # Will raise `ActiveRecord::QueryCanceled`.
    execute <<~SQL
      SELECT pg_sleep(2);
    SQL

    # Resets (disables) timeout.
    execute <<~SQL
      SET statement_timeout = DEFAULT;
    SQL

    # Will not raise.
    execute <<~SQL
      SELECT pg_sleep(2);
    SQL
  end
end

请注意，如果您不调用 disable_ddl_transaction!，您可以使用 SET LOCAL。将两者结合起来是行不通的，因为在事务块之外的 SET LOCAL 没有效果。它记录：

WARNING: SET LOCAL can only be used in transaction blocks

PS：你跟注 disable_ddl_transaction! 会加注 NoMethodError。它应该在 def change 定义之前。

Answer 3

打开 log_duration 并进行更多调查后，我想我得出的结论是 postgres 只是不考虑删除列花费超过 1 毫秒，即使记录了这样的事情：

statement: ALTER TABLE "foos" DROP COLUMN "bar"
duration: 1.171 ms
duration: 0.068 ms
duration: 0.328 ms

但是有了下面的食谱，

我能够让另一种类型的查询因语句超时而失败
由于锁定超时，我能够使删除列失败（在 psql 会话中保持锁定）

class TimeoutTest < ActiveRecord::Migration[5.2]

  def change
    execute <<~SQL
      SET statement_timeout = 1; -- ms
      SET lock_timeout = 1; -- ms
    SQL

    # I can't get this to fail due to a statement timeout, but does fail due to lock timeout
    # interestingly, if both timeouts are set, it will be statement timeout which happens first. not
    # sure if this is a pg bug, or if calculating the lock timeout somehow increases the overall statement
    # overhead
    remove_column :foos, :bar

    # With 100k of rows of test data, this does fail the statement timeout
    Foo.unscoped.update_all(bar: 5)
  end
end

如何为单个迁移设置 statement_timeout？

How can I set statement_timeout for an individual migration?

postgresql

ruby-on-rails

database-migration