DB2 CURSOR

Existence check

Sometimes we need to check if a specific datum is in the table in order to decide what to do.

e.g. If it exists do “A” Else do “B”.

Datum - singular

Data - plural

Existence check -BAD (1)Existence check -BAD (1)

SELECT DISTINCT 1INTO :HVFROM TABLEWHERE ….

1. If there is no index to satisfy the “WHERE”then we do tablespace scan and sort.

2. Even if we have an index, we may have duplicates

so we still invoke the sort.3. Even if we have unique index – it may be changed

in the future.4. SQL code may be copied, bad practice continues.

Existence check –BAD (2)Existence check –BAD (2) SELECT COUNT(*)INTO :HVFROM TABLEWHERE ….

1. If there is no index to satisfy the “WHERE”then we do tablespace scan.

2. Even if we have index, we count all occurrences.

(It’s o.k. If we were asked to count all rows).

Existence check – GOOD (3)Existence check – GOOD (3)

1. DECLARE CURSOR.(Optimize for 1 row with UR)

2. OPEN CURSOR.3. FETCH.4. CLOSE CURSOR.

1. Need to code 4 SQL statements.2. Looks cumbersome.3. Uses more resource then the next idea.

Existence check – GOOD (4)Existence check – GOOD (4)

SELECT 1INTO :HVFROM TABLEWHERE ….

IF SQLCODE = +100 THEN “NOT_EXISTS”IF SQLCODE = 0 or SQLCODE = -811 THEN “EXISTS”.

1. Needs some documentation in the program.2. Don’t use any data returned by it.

Internal secretInternal secret

• A A singletonsingleton select is done internally as a cursor ! select is done internally as a cursor !• Does not use cross memory calls as regular cursor.Does not use cross memory calls as regular cursor.• Internal code length is shorter then regular cursor.Internal code length is shorter then regular cursor.• DB2 builds a cursor read, internally, for the DB2 builds a cursor read, internally, for the

singleton select and does 1 or 2 fetch commands:singleton select and does 1 or 2 fetch commands:– If the 1st command finds nothing,If the 1st command finds nothing,

we get a sqlcode = +100. we get a sqlcode = +100.– If the 2nd command finds something,If the 2nd command finds something,

we get a sqlcode = -811.we get a sqlcode = -811.– Else we get the data and sqlcode = 0.Else we get the data and sqlcode = 0.

Sub Select - BADSub Select - BAD

SELECT * FROM T1WHERE T1.CODE IN

(select T2.code

from T2 where T2.key = ‘X’)

1. Will cause tablespace scan on T1.2. DB2 may change this type of

sub select to Join (if possible).

Do it as join -GOODDo it as join -GOOD

SELECT T1.* FROM T1 , T2WHERE T2.KEY = ‘X’AND T1.CODE=T2.CODE

Sub Select - GOODSub Select - GOOD

SELECT * FROM T1WHERE T1.CODE NOT IN

(select T2.code from T2 where T2.key = ‘X’)

Can’t be done as a Join.

Sub Select - GOODSub Select - GOOD

SELECT * FROM T1WHERE NOT EXISTS

(select 1 from T2 where T1.code=T2.code)

Can’t be done as a Join.

Sub Select – BAD or GOOD ?Sub Select – BAD or GOOD ?

SELECT A1, A2, A3 FROM T1WHERE A1 = ?AND A2 =

(select max(A2) from T1)

Use a cursor - BAD or GOOD ?Use a cursor - BAD or GOOD ?

DECLARE CRS1 CURSOR FORSELECT A1, A2, A3 FROM T1WHERE A1 = ?ORDER BY A2 DESCOPTIMIZE FOR 1 ROW

Open crs1; fetch crs1 into…. ; close crs1

Statistics

Time CPU SQL Sorts Locks Rows

0.00392 0.00341 4 0 7 4 0.00625 0.00517 4 1 9 24

Sub-Select

Cursor

Assuming proper indexin both cases !!!

Sub-Query vs. Cursor - ConclusionSub-Query vs. Cursor - Conclusion

• Assuming proper index:– If the command is used infrequently then we

can use the sub-query, otherwise – use the cursor.

• If no proper index exists:– The cursor will invoke sort on all the rows that

conform to the search criteria.– The sub-query will scan all rows for the

max/min value but will not sort.– Use the sub-query.

Conclusion

Proper indexes can help

Real life example (1)

SELECT * FROM MNTB.TVTNSDRA A WHERE A.LOT_NUMBER IN

(SELECT B.LOT_NUMBER FROM MNTB.TVTNITUR B WHERE UNIT = '638‘ AND

B.LOT_NUMBER = A.LOT_NUMBER);

Canceled after 23 minutes elapsed

Join column not 1st in index

Real life example (2)

SELECT A.* FROM MNTB.TVTNSDRA A,

MNTB.TVTNITUR B WHERE UNIT = '638‘ AND

B.LOT_NUMBER = A.LOT_NUMBER WITH UR ;

Canceled after 14 minutes elapsed

Join column not 1st in index

Real life example (3)

SELECT * FROM MNTB.TVTNSDRA A WHERE A.LOT_NUMBER IN

(SELECT DISTINCT B.LOT_NUMBER FROM MNTB.TVTNITUR B WHERE UNIT = '638‘) WITH UR;

Finished after 14 seconds elapsed

Join column not 1st in index

Why?

• The 1st example is a correlated sub-query where the inner query is executed for every row of the outer query.

• The 2nd example is a join that has no suitable index.

• The 3rd example is a non-correlated sub-query where the inner query is executed only once, the result table is kept sorted in memory and the external query checks against it.

Need a date ?

Select distinct current datefrom table1;

select current datefrom sysibm.sysdummy1;

EXEC SQLSET :HV = CURRENT DATE ;

Sub Select – IN vs. EXISTS (3)

SELECT A, B, C FROM TAB1WHERE EXISTS

(SELECT 1 FROM TAB2 WHERE ……);

OUTER

INNER

Sub Select – IN vs. EXISTS (4)

• If the “inner” table is big or if there is usable index on it then EXISTS will perform better.

• If the “inner” table is small or there is no usable index on it then IN will perform better.

• If there are few rows that qualify then the query will be converted to IN (list) which allows a matching index scan.

SELECT *

• Don’t use “SELECT *” unless you really need all columns.

• Each column has to be moved from DB2 page to the DM, then to the RDS and then to the working program.

• This move is done field by field.

ORDER BY

• Include only the columns needed for the sort.

Select A1, B1, C1From tableWhere A1 = :hv1Order by A1, A2, A3

Select A1, B1, C1From tableWhere A1 = :hv1Order by A2, A3

Cursor within a cursor

• Cursor within a cursor (in program code) means a lot of unnecessary open & close operations of the internal cursor.

• Code it as a join / sub-select / in-list instead.

Divide and conquer

• Teachers table• Courses table• Each teacher can teach any

number of courses.• We look for teachers who can

teach all courses.

DIVIDE (1)

CREATE TABLE DIV1 (KD1 INT NOT NULL, DD1 CHAR(5) NOT NULL);

CREATE TABLE DIV2 (KD2 INT NOT NULL, KD1 INT NOT NULL);

Bring all records from DIV2 which have all occurrences from DIV1.

DIVIDE (2)

1 AAA

2 BBB

3 CCC

100100 11

100100 22

100100 33

101101 11

101101 55

102102 11

102102 22

102102 33

102102 44

104104 11

DIV1

DIV2

Result: 100

KD1 DD1

KD2 KD1

DIVIDE (3)

SELECT A.KD2 FROM (SELECT DISTINCT DIV2.KD2 AS KD2, DIV2.KD1 AS KD1 FROM DIV2 GROUP BY DIV2.KD2, DIV2.KD1) AS A

GROUP BY A.KD2 HAVING COUNT(*) = (SELECT COUNT(*) FROM DIV1) AND NOT EXISTS (SELECT DIV2.KD1 FROM DIV2 WHERE A.KD2=DIV2.KD2 AND DIV2.KD1 NOT IN (SELECT DIV1.KD1 FROM DIV1));

Find Duplicates

SELECT A, B, C, COUNT(*) AS ‘NUM#’FROM T1GROUP BY A, B, CHAVING COUNT(*) > 1[ORDER BY 4 DESC]

GROUP BY ON FUNCTIONS (1)

SELECT DEPT, GROSS_SALARYFROM (SELECT DEPT, SALARY+BONUS

AS GROSS_SALARY FROM EMP WHERE RANK >= 30) AS A

GROUP BY DEPT, GROSS_SALARY

GROUP BY ON FUNCTIONS (2)

SELECT SUM(SALARY), MONTH_SALFROM (SELECT SALARY

,MONTH(SALARY_DATE) AS MONTH_SAL FROM EMP ) AS A

GROUP BY MONTH_SAL

How much (does it costs) ?

Statement typeStatement type Estimated number of machine instructions

Simple FETCHSimple FETCH 3,500 to 9,000

Singleton SELECTSingleton SELECT 12,000 to 40,000

Update/Delete/InsertUpdate/Delete/Insert 40,000 to 90,000

The ENDThe END