PostgreSQL从选择限制插入

但在执行完死刑后我已经插入了257k。

我对这个问题的期望

SELECT count(*) FROM daily_checkups WHERE pig_migration_count = 0

结果是40万

有人能解释一下,为什么插入后我的行数减少了?

insert(<<-SQL.squish)
  INSERT INTO pig_migrations (daily_checkup_id, created_at, updated_at, migration_type, amount)
  (
    SELECT id, created_at, updated_at, 4 as migration_type, 0 as amount
    FROM daily_checkups WHERE pig_migration_count = 0
  )
SQL