SQL-在SELECT中具有多个CASE语句会对性能产生什么影响-Teradata

Question

所以我有一个查询，在SELECT中需要一堆CASE语句。这不是原始的设计，而是折衷的一部分。

因此查询看起来像这样：

SELECT
  CONT.TABLE.FINC_ACCT_NM,
  CONT.TABLE.FINC_ACCT_ID,
  CONT.TABLE.CURR_END_OF_PERD_ACTL_VAL,
  CONT.TABLE.PREV_END_OF_PERD_ACTL_VAL,
  CONT.TABLE.VARNC_PLAN_VAL,
  CONT.TABLE.OUTLOOK_BDGT_PLAN_VAL,
  CONT.TABLE.PERD_END_RPT_DT,
  CONT.TABLE.PLAN_VERS_NM,
  CONT.TABLE.FRMT_ACTL_CD,
  CONT.TABLE.FRMT_PLAN_CD,
  CONT.TABLE.RPT_PERD_TYPE_CD,
  CASE 
                WHEN ( CONT.TABLE.FINC_ACCT_ID )=           'XXXX'        and ( CONT.TABLE.BAL_TYPE_CD ) =             'EOP'      then      '  Net Interest Income'  
                WHEN ( CONT.TABLE.FINC_ACCT_ID )=           'XXXX'        and ( CONT.TABLE.BAL_TYPE_CD ) =             'EOP'      then      '  Non Interest Income'
                WHEN ( CONT.TABLE.FINC_ACCT_ID )=           'XXXX'        and ( CONT.TABLE.BAL_TYPE_CD ) =             'EOP'      then      'Non-Interest Expense'
                WHEN ( CONT.TABLE.FINC_ACCT_ID )=           'XXXX'        and ( CONT.TABLE.BAL_TYPE_CD ) =             'EOP'      then      '  Total Marketing Expense'
                WHEN ( CONT.TABLE.FINC_ACCT_ID )=           'XXXX'        and ( CONT.TABLE.BAL_TYPE_CD ) =             'EOP'      then      '  Total Operating Expense'
                WHEN ( CONT.TABLE.FINC_ACCT_ID )=           'XXXX'        and ( CONT.TABLE.BAL_TYPE_CD ) =             'EOP'      then      'Pre-Provision Earnings (before tax)'
                WHEN ( CONT.TABLE.FINC_ACCT_ID )=           'XXXX'        and ( CONT.TABLE.BAL_TYPE_CD ) =             'EOP'      then      '  Net Charge-offs'
                WHEN ( CONT.TABLE.FINC_ACCT_ID )=           'XXXX'        and ( CONT.TABLE.BAL_TYPE_CD ) =             'EOP'      then      '  Other'
                WHEN ( CONT.TABLE.FINC_ACCT_ID )=           'XXXX'        and ( CONT.TABLE.BAL_TYPE_CD ) =             'EOP'      then      '  Allowance Build (Release)'
                WHEN ( CONT.TABLE.FINC_ACCT_ID )=           'XXXX'        and ( CONT.TABLE.BAL_TYPE_CD ) =             'EOP'      then      'Provision Expense'
                WHEN ( CONT.TABLE.FINC_ACCT_ID )=           'XXXX'        and ( CONT.TABLE.BAL_TYPE_CD ) =             'EOP'      then      'Pretax Income'
                WHEN ( CONT.TABLE.FINC_ACCT_ID )=           'XXXX'        and ( CONT.TABLE.BAL_TYPE_CD ) =             'EOP'      then      'Tax Expense'
                WHEN ( CONT.TABLE.FINC_ACCT_ID )=           'XXXX'        and ( CONT.TABLE.BAL_TYPE_CD ) =             'EOP'      then      'NIAT'
                WHEN ( CONT.TABLE.FINC_ACCT_ID )=           'XXXX'        and ( CONT.TABLE.BAL_TYPE_CD ) =             'EOP'      then      'EPS'
                WHEN ( CONT.TABLE.FINC_ACCT_ID )=           'XXXX'        and ( CONT.TABLE.BAL_TYPE_CD ) =             'EOP'      then      'Ending Loans - HFI'
                WHEN ( CONT.TABLE.FINC_ACCT_ID )=           'XXXX'        and ( CONT.TABLE.BAL_TYPE_CD ) =             'avg'       then      'Average Loans - HFI'
                WHEN ( CONT.TABLE.FINC_ACCT_ID )=           'XXXX'        and ( CONT.TABLE.BAL_TYPE_CD ) =             'avg'       then      'Average Earning Assets'
                WHEN ( CONT.TABLE.FINC_ACCT_ID )=           'XXXX'        and ( CONT.TABLE.BAL_TYPE_CD ) =             'EOP'      then      'Ending Deposits'
                WHEN ( CONT.TABLE.FINC_ACCT_ID )=           'XXXX'        and ( CONT.TABLE.BAL_TYPE_CD ) =             'avg'       then      'Average Deposits'
                WHEN ( CONT.TABLE.FINC_ACCT_ID )=           'XXXX'        and ( CONT.TABLE.BAL_TYPE_CD ) =             'EOP'      then      'NIM on Loans'
                WHEN ( CONT.TABLE.FINC_ACCT_ID )=           'XXXX'        and ( CONT.TABLE.BAL_TYPE_CD ) =             'EOP'      then      'Revenue Margin'
                WHEN ( CONT.TABLE.FINC_ACCT_ID )=           'AC579'        and ( CONT.TABLE.BAL_TYPE_CD ) =             'EOP'      then      'Charge off rate'
                WHEN ( CONT.TABLE.FINC_ACCT_ID )=           'XXXX'        and ( CONT.TABLE.BAL_TYPE_CD ) =             'EOP'      then      'Efficiency ratio'
                WHEN ( CONT.TABLE.FINC_ACCT_ID )=           'XXXX'        and ( CONT.TABLE.BAL_TYPE_CD ) =             'EOP'      then      'ROA'
                WHEN ( CONT.TABLE.FINC_ACCT_ID )=           'XXXX'        and ( CONT.TABLE.BAL_TYPE_CD ) =             'EOP'      then      'ROE'
                WHEN ( CONT.TABLE.FINC_ACCT_ID )=           'XXXX'        and ( CONT.TABLE.BAL_TYPE_CD ) =             'EOP'      then      'Return on Allocated Capital (ROAC)'



  ELSE ( CONT.TABLE.FINC_ACCT_NM ) end
FROM
  CONT.TABLE
WHERE
  (
   (
    ( ( CONT.TABLE.PERD_END_RPT_DT ) = (

SELECT Max(Perd_END_RPT_DT) 
FROM CONT.TABLE
Where VERS_NM='Actual'
   AND RPT_PERD_TYPE_CD = 'Q'
   AND DATA_VLDTN_IND='Y'
)
   AND RPT_PERD_TYPE_CD = 'Q'
  AND DATA_VLDTN_IND='Y'  )
    OR
    ( ( CONT.TABLE.PERD_END_RPT_DT ) = (

SELECT Max(Perd_END_RPT_DT) 
FROM CONT.TABLE
Where VERS_NM='Actual'
   AND RPT_PERD_TYPE_CD = 'M'
   AND DATA_VLDTN_IND='Y'
)

  AND RPT_PERD_TYPE_CD = 'M'
  AND DATA_VLDTN_IND='Y'  )
   )
   AND
   ( ( CONT.TABLE.DATA_VLDTN_IND )='Y'  )
   AND
   ( ( CONT.TABLE.FINC_ACCT_ID )IN ('AC0006470','AC8000199','AC8002145','AC0006586','AC8000094')  AND ( CONT.TABLE.DEPT_ID )='OR80637'  )
  )

我的问题是，将所有这些CASE语句更改为直接列引用会对性能产生什么影响。

换句话说：如果我将每个CASE语句更改为一个列名，并从查询中删除了所有CASE语句，那么会对性能产生很大影响，为什么？

我正在对此进行测试，因此我可以弄清楚性能是否受到影响，但是我对WHY的细节也同样感兴趣？（为什么的技术细节）

谢谢你的帮助！

Answer 1

与WHERE子句中的联接相比，case语句的影响要小得多。

SQL性能的主要驱动力是I / O －从磁盘读取数据。我认为它比行处理要重要两个数量级。这只是一种启发式方法，并不基于数据库上的特定测试。

您正在执行自联接，这将需要大量的工作来读取表或进行大量的索引处理工作。

另一方面，case语句变成了非常原始的硬件命令-
equals，gotos等。数据驻留在最靠近处理器的内存中，因此它将进行压缩。您对case语句（例如like或子查询）没有任何幻想。我可以想象，如果删除语句中的大多数行，查询的速度将一样快。

如果您对性能有疑问，请在（VERS_NM，RPT_PERD_TYPE_CD，DATA_VLDTN_IND，Perd_END_RPT_DT）上添加索引。这个由四部分组成的索引应该使您能够获得最长时间，而无需在原始表上调用I
/ O请求。