SQL - What is the performance impact of having multiple CASE statements in SELECT - Teradata

Question

So I have a query that requires a bunch of CASE statements in the SELECT. This was not the orginal design but part of a compromise.

So the query looks something like this:

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'  )
  )

My question is what affect would changing all those CASE statements to direct column references have on performance.

In other words: If I changed every CASE statement to just a column name and removed all CASE statements from the query would there be a large impact on performance and why?

I am testing this out so I can figure out if performance is affected but I am just as interested in the details of WHY? (Technical details of why)

Thanks for your help!

Why not use a JOIN? In any case, the way to "find out" would be to run the different queries and then look at the execution plans .. (this should be done *before* asking such an open question on SO; a little bit more information will make the question not-needed and/or more focused/interesting) — , Aug 17 '12 at 20:35
Why don't you change the select clause to just `SELECT CONT.TABLE.FINC_ACCT_NM ` and see how it compares. If there's little or no difference than you'll know you need to look elsewhere — Conrad Frix, Aug 17 '12 at 21:15

score 30 · Accepted Answer · edited Aug 17 '12 at 21:07

The case statements are going to be much less of a factor than the joins in the WHERE clause.

The main driver of performance in SQL is I/O -- reading the data from disk. I think of it as two orders of magnitude more important than the processing going on in rows. This is just a heuristic, not based on specific tests on a database.

You are doing self-joins, which will require either lots of work reading the table or a fair amount of work dealing with indexes.

The case statement, on the other hand, gets turned into very primitive hardware commands -- equals, gotos, and the like. The data resides in memory closest to the processors, so it is going to zip along. You are doing nothing fancy in the case statement (such as a like or a subquery). I would imagine that the query would be just as fast if you removed most of the lines in the statement.

If you are having issues with performance, put an index on (VERS_NM, RPT_PERD_TYPE_CD, DATA_VLDTN_IND, Perd_END_RPT_DT). This four-part index should allow you to get the max date without invoking I/O requests on the original table.

bhamby · Answer 2 · 2012-08-17T21:24:36.643

Edit: Actually, you can re-factor both of those sub-queries into a JOIN, which would probably be faster, anyway. It gets rid of a lot of repetition, too!

This really isn't about the performance of the query ( @Gordon has that covered pretty well), but that huge case statement just seems like a maintenance nightmare. Maybe a better way to handle that would be to convert it to a table

CREATE TABLE ACCT_DISPLAY_NAME (
    FINC_ACCT_ID CHAR(10),
    BAL_TYPE_CD  CHAR(3),
    DISPLAY_NAME VARCHAR(100)
);

CREATE INDEX ACCT_DISPLAY_INDEX ON ACCT_DISPLAY_NAME (
    FINC_ACCT_ID,
    BAL_TYPE_CD
);

INSERT INTO ACCT_DISPLAY_NAME VALUES
('AC99800'  , 'EOP', '  Net Interest Income'               ),
('AC12993'  , 'EOP', '  Non Interest Income'               ),
('AC667999' , 'EOP', 'Non-Interest Expense'                ),
('AC996587' , 'EOP', '  Total Marketing Expense'           ),
('AC659986' , 'EOP', '  Total Operating Expense'           ),
('AC69678'  , 'EOP', 'Pre-Provision Earnings (before tax)' ),
('AC09994'  , 'EOP', '  Net Charge-offs'                   ),
('AC20977'  , 'EOP', '  Other'                             ),
('AC19979'  , 'EOP', '  Allowance Build (Release)'         ),
('AC7094'   , 'EOP', 'Provision Expense'                   ),
('AC6997'   , 'EOP', 'Pretax Income'                       ),
('AC0994'   , 'EOP', 'Tax Expense'                         ),
('AC9999'   , 'EOP', 'NIAT'                                ),
('AC7990'   , 'EOP', 'EPS'                                 ),
('AC9995'   , 'EOP', 'Ending Loans - HFI'                  ),
('AC9995'   , 'avg', 'Average Loans - HFI'                 ),
('AC2991'   , 'avg', 'Average Earning Assets'              ),
('AC2999'   , 'EOP', 'Ending Deposits'                     ),
('AC9999'   , 'avg', 'Average Deposits'                    ),
('AC0379'   , 'EOP', 'NIM on Loans'                        ),
('AC6999'   , 'EOP', 'Revenue Margin'                      ),
('AC579'    , 'EOP', 'Charge off rate'                     ),
('AC5899'   , 'EOP', 'Efficiency ratio'                    ),
('AC629'    , 'EOP', 'ROA'                                 ),
('AC359'    , 'EOP', 'ROE'                                 ),
('AC619'    , 'EOP', 'Return on Allocated Capital (ROAC)'  );

And do a LEFT JOIN on it (since you have that ELSE in the CASE), something like:

SELECT T.FINC_ACCT_NM,
       T.FINC_ACCT_ID,
       T.CURR_END_OF_PERD_ACTL_VAL,
       T.PREV_END_OF_PERD_ACTL_VAL,
       T.VARNC_PLAN_VAL,
       T.OUTLOOK_BDGT_PLAN_VAL,
       T.PERD_END_RPT_DT,
       T.PLAN_VERS_NM,
       T.FRMT_ACTL_CD,
       T.FRMT_PLAN_CD,
       T.RPT_PERD_TYPE_CD,
       COALESCE(N.DISPLAY_NAME, T.FINC_ACCT_NM)

FROM CONT.TABLE T
JOIN (
    SELECT RPT_PERD_TYPE_CD, DATA_VLDTN_IND, Max(Perd_END_RPT_DT) AS PERD_END_RPT_DT
    FROM CONT.TABLE
    WHERE VERS_NM='Actual'
      AND DATA_VLDTN_IND='Y'
    GROUP BY RPT_PERD_TYPE_CD, DATA_VLDTN_IND
) AS MAX_DATES
  ON T.RPT_PERD_TYPE_CD = MAX_DATES.RPT_PERD_TYPE_CD
 AND T.DATA_VLDTN_IND   = MAX_DATES.DATA_VLDTN_IND 
 AND T.PERD_END_RPT_DT  = MAX_DATES.PERD_END_RPT_DT 

LEFT JOIN ACCT_DISPLAY_NAME N
  ON T.FINC_ACCT_ID = N.FINC_ACCT_ID
 AND T.BAL_TYPE_CD  = N.BAL_TYPE_CD

WHERE T.DEPT_ID = 'OR80637'

  AND T.RPT_PERD_TYPE_CD IN ('Q', 'M')

  AND T.FINC_ACCT_ID IN (
    'AC0006470',
    'AC8000199',
    'AC8002145',
    'AC0006586',
    'AC8000094'
  )

This has the added advatage of ensuring that any other process that uses the same logic and use the table too will change if you add a new record, rather than searching through 1000 stored procs to find the ones that might need to be changed. Espcially since the data looks as if the type that might change relatively often. — HLGEM, Aug 17 '12 at 21:15
@Ben, I just edited the question and actually re-factored most of that out after noticing some repetition. — bhamby, Aug 17 '12 at 21:19

SQL - What is the performance impact of having multiple CASE statements in SELECT - Teradata

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