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Oracle SQL Hashing Tips

Oracle Tips by Burleson Consulting

A Matter of Hashing

We have discussed hashing in prior lessons, essentially each SQL statement is hashed and this hash value is then used to compare to already stored SQL areas, if a matching hash is found the statements are compared.

The hash is only calculated based on the first 200 or so characters in the SQL statement, so extremely long SQL statements can result in multiple hashes being the same even though the stored SQL is different (if the first 100 or so characters in each statement are identical). This is another argument for using stored procedures and functions to perform operations and for the use of bind variables. There is hope, in 8i  (or 8.1 if you prefer) the hash value will be calculated on the first 100 and last 100 characters reducing the chances of multiple identical hash values for different SQL statements.

If the number of large, nearly identical statements is high, then the number of times the parser has to compare a new SQL statement to existing SQL statements with the same hash value increases. This results in a higher statement overhead and poorer performance. You should identify these large statements and encourage users to re-write them using bind variables or to proceduralize them using PL/SQL. The report in Source 24 will show if you have a problem with multiple statements being hashed to the same value.

rem: FUNCTION: Shows by user who has possible
rem:           SQL reuse problems
COLUMN total_hash                         HEADING 'Total Hash|Values'
COLUMN same_hash                    HEADING 'SQL With|Same Hash'
COLUMN u_hash_ratio     FORMAT 999.999    HEADING 'SQL Sharing|Hash'
START title80 'Shared Hash Value Report'
SPOOL rep_out\&&db\shared_hash.lst
BREAK ON REPORT
COMPUTE SUM OF total_hash ON REPORT
COMPUTE SUM OF same_hash ON REPORT
SELECT
      a.username,
      count(b.hash_value) total_hash,
      count(b.hash_value)-COUNT(UNIQUE(b.hash_value)) same_hash,
(COUNT(UNIQUE(b.hash_value))/COUNT(b.hash_value))*100 u_hash_ratio
FROM
      dba_users a, v$sqlarea b
WHERE
      a.user_id=b.parsing_user_id
GROUP BY
      a.username;
CLEAR COMPUTES

Source 24: Example Script to Report on Hashing Problems

The script in Source 24 produces a report similar to that shown in Listing 23. The report in Listing 23 shows which users are generating SQL that hashes to the same values. Once you have a user isolated you can then run the script in Source 12 to find the bad SQL statements.

Date: 11/20/98                                              Page:   1
Time: 11:40 AM            Shared Hash Value Report          AULTM
                              DCARS database

                               Total Hash  SQL With SQL Sharing
USERNAME                           Values Same Hash        Hash
------------------------------ ---------- --------- -----------
AULTM                                 129         0     100.000
DCARS                                6484        58      99.105
QDBA                                  109         0     100.000
RCAPS                                 270         0     100.000
RCOM                                  342         7      97.953
SECURITY_ADMIN                         46         0     100.000
SYS                                   134         0     100.000
                               ---------- ---------
sum                                  7514        65


Listing 23: Hash Report

A quick glance at the report in Listing 23 shows that I need to look at the DCARS user to correct  the hashing problems they might be having and improve the reuse of SQL in the shared pool. However, look at the number of hash areas this user has accumulated, 6,484, if I run the report from Source 12 in the first half of this lesson it will out weigh the paper version of the Oracle documentation set. A faster way to find the hash values would be to do a self-join and filter out the hash values that are duplicated. Sounds easy enough, but remember, the V$ tables have no ROWIDs so you can?t use the classic methods, you have to find another column that will be different when the HASH_VALUE column in V$SQLAREA is the same. Look at the select in Source 25.

select distinct a.hash_value from v$sqlarea a, v$sqlarea b, dba_users c
where a.hash_value=b.hash_value and
a.parsing_user_id = c.user_id
and c.username='DCARS' and  <-- change to user you are concerned about
a.FIRST_LOAD_TIME != b.FIRST_LOAD_TIME

Source 25: Example Select To Determine Duplicate Hash Values

Listing 24 has an example output from the above select.

DCARS:column hash_value format 99999999999
DCARS:set echo on
DCARS: select distinct a.hash_value from v$sqlarea a, v$sqlarea b,
  2  dba_users c
  3  where a.hash_value=b.hash_value and
  4  a.parsing_user_id = c.user_id
  5  and c.username='DCARS' and
  6* a.FIRST_LOAD_TIME != b.FIRST_LOAD_TIME

  HASH_VALUE
------------
 -1595172473
 -1478772040
 -1344554312
  -941902153
  -807684425
  -507978165
  -270812489
   441376718
   784076104
   979296206
  1765990350
  1945885214

Listing 24: Example Hash Select Output

Once you have the hash value you can pull the problem SQL statements from either V$SQLAREA or V$SQLTEXT very easily, look at Listing 25.

DCARS:select sql_text from v$sqlarea where hash_value='441376718';

SQL_TEXT
-----------------------------------------------------------------------
SELECT   region_code,   region_dealer_num,   consolidated_dealer_num,  
dealer_name,   dealer_status_code,   dealer_type_code,   mach_credit_code,
parts_credit_code  FROM dealer  WHERE   region_code = '32' AND   reg_dealer_num = '6433'


SELECT   region_code,   region_dealer_num,   consolidated_dealer_num,  
dealer_name,   dealer_status_code,   dealer_type_code,   mach_credit_code,
parts_credit_code  FROM dealer  WHERE   region_code = '56' AND   reg_dealer_num = '6273'

Listing 25: Example of Statements With Identical Hash Values But Different SQL

Long statements require special care to see that bind variables are used to prevent this problem with hashing. Another help for long statements is to use views to store values at an intermediate state thus reducing the size of the variable portion of the SQL. Notice in the example select  in Listing 25 that the only difference between the two identically hashed statements is that the ?region_code? and ?reg_dealer_num? comparison values are different, if bind variables had been used in these statements there would only have been one hash entry instead of two.

Guideline 6: Use bind variables, PL/SQL (procedures or functions) and views to reduce the size of large SQL statements to prevent hashing problems.

 


This is an excerpt from the eBook "Oracle DBA made Simple".

For more details on Oracle database administration, see the "Easy Oracle Jumpstart" by Robert Freeman and Steve Karam.  It?s only $19.95 when you buy it directly from the publisher here.

 


 

 
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