DB2 Subquery Removal Test 1

Does DB2 do subquery removal? Let's do a test and find out:

We will use a simplified TPC-H query, and faking 100000 rows for both tables:

Original Statement:
------------------
SELECT
  SUM(l_extendedprice) / 7.0 AS avg_yearly
FROM
  lineitem,
  part
WHERE
  p_partkey = l_partkey AND
  p_brand = '[BRAND]' AND
  p_container = '[CONTAINER]' AND
  l_quantity <
  (SELECT
     0.2 * AVG(l_quantity)
   FROM
     lineitem
   WHERE
     l_partkey = p_partkey
  )



The outcome? See the optimized statement:

Optimized Statement:
-------------------
SELECT
  (Q7.$C0 / 7.0) AS "AVG_YEARLY"
FROM
  (SELECT
     SUM(DECFLOAT(Q6.L_EXTENDEDPRICE, 34))
   FROM
     (SELECT
        Q5.L_EXTENDEDPRICE
      FROM
        (SELECT
           SUM(DECFLOAT(Q2.L_QUANTITY, 34)),
           COUNT(DECFLOAT(Q2.L_QUANTITY, 34))
         FROM
           (SELECT
              Q1.L_QUANTITY
            FROM
             LINEITEM AS Q1
            WHERE
              (Q1.L_PARTKEY = Q4.P_PARTKEY)
           ) AS Q2
        ) AS Q3,
        PART AS Q4,
        LINEITEM AS Q5
      WHERE
        (Q5.L_QUANTITY < (0.2 * (Q3.$C0 / Q3.$C1))) AND
        (Q4.P_CONTAINER = '[CONTAINER]') AND
        (Q4.P_BRAND = '[BRAND]') AND
        (Q4.P_PARTKEY = Q5.L_PARTKEY)
     ) AS Q6
  ) AS Q7




Notice that the subquery has vanished, but LINEITEM Q1 is still being used as a lateral view.
Can it do better than this? Can it reduce the access to LINEITEM from 2 to 1?

More on this later ....

"<> ALL" Multi-column Problem

For single column, everything is fine, but when you try multi-column set comparison, 'it' gets confused.

Let's start with a single column comparison:


db2 => select * from no_null where c1 <> all ( select c1 from some_null) ;

C1                       C2                    
------------------------ ------------------------

  0 record(s) selected.


So far so good.

Now, let's try two columns, c1 and c2:



db2 => select * from no_null where (c1, c2) <>ALL ( select c1, c2 from some_null) ;

SQL0104N  An unexpected token "(" was found following "".  Expected tokens may
include:  "".  SQLSTATE=42601

Fun eh?  ;)

VARCHAR, Trailing Spaces, and Unique Constraint

Another look at VARCHAR. This time, the implication of the 'important-ness' of trailing space(s) and constraint.

Let's start from a table with 3 rows:

db2 => select c1, length(c1) from smallt ;

C1   2
---- -----------
B              1
B              2
B              3

  3 record(s) selected.


As you can see, these three rows are 'unique' in the sense that they have different length. In Oracle, they are three distinct rows with all the 'expected' properties.

Can we create a unique index on it?

Let's try:


db2 => create unique index uqi on smallt (c1 );
DB21034E  The command was processed as an SQL statement because it was not a
valid Command Line Processor command.  During SQL processing it returned:
SQL0603N  A unique index cannot be created because the table contains data
that would result in duplicate index entries.  SQLSTATE=23515


NOPE!

Another example of the 'useless' trailing spaces in VARCHAR.

VARCHAR and Equality Comparison

This is a follow-up on the previous post.

Now that we know trailing space(s) is 'not that important' as far as DB2 is concerned, what would you expect in a simple '=' comparison?

Let's do a little test:



db2 => create table smallt (c1 varchar(4)) ;
DB20000I  The SQL command completed successfully.

db2 => insert into smallt values 'B', 'B ', 'B  ';
DB20000I  The SQL command completed successfully.

db2 => select length(c1), c1 from smallt ;

1           C1
----------- ----
          1 B
          2 B
          3 B

  3 record(s) selected.


We have three rows with different 'length' due to the trailing space(s) we inserted. If we compare this column to itself, ie, c1 = c1, how many rows would you expect?

Three?

Let's see ...



db2 => select * from smallt t1, smallt t2 where t1.c1 = t2.c1 ;

C1   C1
---- ----
B    B
B    B
B    B
B    B
B    B
B    B
B    B
B    B
B    B

  9 record(s) selected.


Need I say more? ;)

Varchar: When a limit is unlimited

How much do you know about 'varchar' type in DB2?

I thought I knew, until I ran a little test and got a nasty surprise.

Here's a simple test case to illustrate:


db2 => create table smallt ( c1 varchar(1));
DB20000I  The SQL command completed successfully.

db2 => insert into smallt values('B             ') ;
DB20000I  The SQL command completed successfully.


db2 => select * from smallt ;

C1
--
B

  1 record(s) selected.



See the 'problem'?


This behaviour can be very disturbing for those used to Oracle DB. Another potential 'hidden' bug for Oracle developer doing DB2.

BLEVEL: How High Can You Go?

While investigating index splits, I encountered a fairly 'mind-numbing' number. But first, here's the basic set up on a database with default page size 4K:

CREATE TABLE LONGT (V1 CHAR(254)) ;
CREATE INDEX IDX_LT ON LONGT ( V1);

And populate this table with 50,000 rows of ascending 'numbers', starting with 1.

Guess what BLEVEL you get?

Drumroll please ... it is ... 5!

I'm beginning to wonder if this BLEVEL has the same meaning as the BLEVEL I have been exposed to ... ;)

Optimizer: Fake it like you mean it

There is another useful utility that lets you fool the optimizer into thinking it has more resource than there is in the hardware. The utility is db2fopt.

This is useful if you are testing the optimizer in a test environment that is considerably less powerful than the production environment. eg, the sortheap in the production system might have 80000 pages, but your tiny test machine might max out at 2000. What do you do then? You can use db2fopt to fake it:

db2fopt <database> update opt_sortheap 80000

Easy.

It is a bit unfortunate that only these 4 are available to be 'faked' as of this writing ( for DB2 LUW v10)

• opt_buffpage
• opt_sortheap
• opt_locklist
• opt_maxlocks