Postgresql 如何在sql语句中选择临时变量

我有以下sql语句：

select * from funds_balance where account_id = 7 and sequence_number <= (
   select sequence_number from funds_balance where account_id = 7 and update_settlement_pool_id = 7
   union all
   select sequence_number from funds_balance_history where account_id = 7 and update_settlement_pool_id = 7
   limit 1
)
union all
select account_id, deliverable_id, cash, credit,pending,sequence_number, update_time,update_reason, update_funds_action_id, update_trade_serial, update_settlement_pool_id
from funds_balance_history where account_id = 7 and sequence_number <= (
   select sequence_number from funds_balance where account_id = 7 and update_settlement_pool_id = 7
   union all
   select sequence_number from funds_balance_history where account_id = 7 and update_settlement_pool_id = 7
   limit 1
)
order by sequence_number desc limit 2

在上面的复杂语句中，有两个相同的子查询，我假设它们执行了两次

有没有办法只执行此子查询一次，或者将其存储为某种temp变量以在外部查询中引用

下面的伪代码描述了我的意思：

var seq_num = select sequence_number from funds_balance where account_id = 7 and update_settlement_pool_id = 7
   union all
   select sequence_number from funds_balance_history where account_id = 7 and update_settlement_pool_id = 7
   limit 1

select * from funds_balance where account_id = 7 and sequence_number <= seq_num
union all
select account_id, deliverable_id, cash, credit,pending,sequence_number, update_time,update_reason, update_funds_action_id, update_trade_serial, update_settlement_pool_id
from funds_balance_history where account_id = 7 and sequence_number <= seq_num
order by sequence_number desc limit 2

---

要简单地将预期行为转换为SQL查询，可以使用：

WITH seq_num AS (

   select sequence_number from funds_balance where account_id = 7 and update_settlement_pool_id = 7
   union all
   select sequence_number from funds_balance_history where account_id = 7 and update_settlement_pool_id = 7
   limit 1

)

select * from funds_balance where account_id = 7 
    and sequence_number <= (SELECT sequence_number FROM seq_num)
union all
select account_id, deliverable_id, cash, credit,pending,sequence_number, update_time,update_reason, update_funds_action_id, update_trade_serial, update_settlement_pool_id
from funds_balance_history where account_id = 7 
    and sequence_number <= (SELECT sequence_number FROM seq_num)

---

我认为我的子查询执行了两次，对吗？或者查询优化人员看到了吗？我相信这取决于您的数据库配置和表结构。Tbh，我不太确定，但是当您显示执行计划时，您可以看到：解释分析。解释分析显示，使用您的方法，计划时间和执行时间都减少了50%。