sql - 使用 in (select ...) 子句进行查询优化

Question

我在 Windows 上使用 Firebird WI-V3.0.4.33054。

我在优化此查询时遇到问题，它使用带有选择的 in 子句:

update CADPC p set p.STA = 'L'
where p.COD in (select distinct CODPC from CADPCI_Rec where IDNfr = 27)
and not exists (select * from CADPCI where CODPC = p.COD)

此查询的计划是(明显的问题是 P NATURAL 部分):

PLAN SORT (CADPCI_REC INDEX (PK_CADPCI_REC))
PLAN (CADPCI INDEX (FK_CADPCI_CODPC))
PLAN (P NATURAL)

Select Expression
    -> Filter
        -> Unique Sort (record length: 36, key length: 8)
            -> Filter
                -> Table "CADPCI_REC" Access By ID
                    -> Bitmap
                        -> Index "PK_CADPCI_REC" Range Scan (partial match: 1/3)
Select Expression
    -> Filter
        -> Table "CADPCI" Access By ID
            -> Bitmap
                -> Index "FK_CADPCI_CODPC" Range Scan (full match)
Select Expression
    -> Filter
        -> Table "CADPC" as "P" Full Scan

另一方面，如果我手动运行 select distinct，复制结果并粘贴到查询中，如下所示:

update CADPC p set p.STA = 'L'
where p.COD in (5699, 5877, 5985)
and not exists (select * from CADPCI where CODPC = p.COD)

现在优化器为 P 表选择了一个合理的计划，查询运行得非常快:

PLAN (CADPCI INDEX (FK_CADPCI_CODPC))
PLAN (P INDEX (PK_CADPC, PK_CADPC, PK_CADPC))

Select Expression
    -> Filter
        -> Table "CADPCI" Access By ID
            -> Bitmap
                -> Index "FK_CADPCI_CODPC" Range Scan (full match)
Select Expression
    -> Filter
        -> Table "CADPC" as "P" Access By ID
            -> Bitmap Or
                -> Bitmap Or
                    -> Bitmap
                        -> Index "PK_CADPC" Unique Scan
                    -> Bitmap
                        -> Index "PK_CADPC" Unique Scan
                -> Bitmap
                    -> Index "PK_CADPC" Unique Scan

我也尝试过在这两种情况下都存在，但结果是一样的:子查询对每一行都重新求值。

update CADPC p set p.STA = 'L'
where exists (select * from CADPCI_Rec where IDNfr = 27 and CODPC = p.COD)
and not exists (select * from CADPCI where CODPC = p.COD)

计划:

PLAN (CADPCI_REC INDEX (PK_CADPCI_REC))
PLAN (CADPCI INDEX (FK_CADPCI_CODPC))
PLAN (P NATURAL)

Select Expression
    -> Filter
        -> Table "CADPCI_REC" Access By ID
            -> Bitmap
                -> Index "PK_CADPCI_REC" Range Scan (partial match: 1/3)
Select Expression
    -> Filter
        -> Table "CADPCI" Access By ID
            -> Bitmap
                -> Index "FK_CADPCI_CODPC" Range Scan (full match)
Select Expression
    -> Filter
        -> Table "CADPC" as "P" Full Scan

因此，问题是:当 in 子句包含选择(通常只有几条记录)时，我能否以某种方式让引擎选择索引计划？

Answer 1

您可以尝试EXECUTE BLOCK 和“反转控制”

• https://firebirdsql.org/file/documentation/reference_manuals/fblangref25-en/html/fblangref25-dml-execblock.html
• https://firebirdsql.org/file/documentation/reference_manuals/fblangref25-en/html/fblangref25-psql-coding.html#fblangref25-psql-forselect

本质上是发布一个匿名的临时存储过程

EXECUTE BLOCK AS
  declare id INTEGER;
BEGIN
  for select distinct t1.CODPC from CADPCI_Rec t1 
    left join CADPCI t2 on where t2.CODPC = t1.CODPC
  where t2.CODPC is NULL and t1.IDNfr = 27
  into :id
  do 
    update CADPC p set p.STA = 'L' where p.COD = :ID and p.STA <> 'L';
END

---

您还可以使用全局临时表 (GTT)

• https://firebirdsql.org/refdocs/langrefupd21-ddl-table.html

然后在实际删除之前创建 ID 列表。

数据库准备(创建无主体表):

CREATE GLOBAL TEMPORARY TABLE CADPC_mark_IDs
   ( ID integer )
ON COMMIT DELETE ROWS

然后命令就像

insert into CADPC_mark_IDs(ID)
select distinct t1.CODPC from CADPCI_Rec t1 
   left join CADPCI t2 on where t2.CODPC = t1.CODPC
where t2.CODPC is NULL and t1.IDNfr = 27

然后

update CADPC p set p.STA = 'L'
where p.COD in (select * from CADPC_mark_IDs) and p.STA <> 'L'

然后

commit; -- clear the in-memory table for next uses

---

在将“不存在的地方”转换为“左连接”(已在上面完成，希望是正确的)之后，还有一个选项，如 Mark 所建议的那样，将使用 MERGE。

• https://firebirdsql.org/refdocs/langrefupd21-merge.html

一些东西

merge into CADPC p
  using (
    select distinct t1.CODPC as id from CADPCI_Rec t1 
      left join CADPCI t2 on where t2.CODPC = t1.CODPC
    where t2.CODPC is NULL and t1.IDNfr = 27
  ) t
on (t.id = p.COD) and (p.STA <> 'L')
when matched then update set p.STA = 'L'