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Clemens Bleile

Oracle FAST=TRUE in sqlplus? Some thoughts about rowprefetch

During my time as a Consultant working on Tuning Tasks I had the feeling that many people think that there is an Oracle-parameter “FAST=TRUE” to speed up the performance and throughput of the database calls. Unfortunately such a parameter is not available, but since version 12cR2 Oracle provided the option “-F” or “-FAST” for sqlplus, which looks like a “FAST=TRUE”-setting. Here an excerpt from the documentation:


The FAST option improves general performance. This command line option changes the values of the following default settings:
 
- ARRAYSIZE = 100
- LOBPREFETCH = 16384
- PAGESIZE = 50000
- ROWPREFETCH = 2
- STATEMENTCACHE = 20

I was interested in where the rowprefetch-setting could result in an improvement.

The documentation about rowprefetch is as follows:


SET ROWPREFETCH {1 | n}
 
Sets the number of rows that SQL*Plus will prefetch from the database at one time. The default value is 1.
 
Example
 
To set the number of prefetched rows to 200, enter
 
SET ROWPREFETCH 200
 
If you do not specify a value for n, the default is 1 row. This means that rowprefetching is off.
 
Note: The amount of data contained in the prefetched rows should not exceed the maximum value of 2147483648 bytes (2 Gigabytes). The  setting in the oraaccess.xml file can override the SET ROWPREFETCH setting in SQL*Plus. For more information about oraaccess.xml, see the Oracle Call Interface Programmer's Guide. 

A simple test where rowprefetch can make a difference is the use of hash clusters (see the Buffers column in the execution plan below). E.g.


SQL> create cluster DEMO_CLUSTER(CUST_ID number) size 4096 single table hashkeys 1000 ;
 
Cluster created.
 
SQL> create table DEMO cluster DEMO_CLUSTER(CUST_ID) as select * from CUSTOMERS;
 
Table created.
 
SQL> exec dbms_stats.gather_table_stats(user,'DEMO');
 
PL/SQL procedure successfully completed.
 
SQL> select num_rows,blocks from user_tables where table_name='DEMO';
 
  NUM_ROWS     BLOCKS
---------- ----------
     55500	 1035
 
SQL> show rowprefetch
rowprefetch 1
SQL> select /*+ gather_plan_statistics */ rowid,cust_id from DEMO where cust_id=101;
 
ROWID		      CUST_ID
------------------ ----------
AAAR4qAAMAAAAedAAA	  101
 
SQL> select * from table(dbms_xplan.display_cursor(format=>'allstats last'));
 
PLAN_TABLE_OUTPUT
-----------------------------------------
SQL_ID	9g2nyr9h2ytk4, child number 0
-------------------------------------
select /*+ gather_plan_statistics */ rowid,cust_id from DEMO where
cust_id=101
 
Plan hash value: 3286081706
 
------------------------------------------------------------------------------------
| Id  | Operation	  | Name | Starts | E-Rows | A-Rows |	A-Time	 | Buffers |
------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |	 |	1 |	   |	  1 |00:00:00.01 |	 2 |
|*  1 |  TABLE ACCESS HASH| DEMO |	1 |	 1 |	  1 |00:00:00.01 |	 2 |
------------------------------------------------------------------------------------
 
Predicate Information (identified by operation id):
---------------------------------------------------
 
   1 - access("CUST_ID"=101)
 
SQL> set rowprefetch 2
SQL> select /*+ gather_plan_statistics */ rowid,cust_id from DEMO where cust_id=101;
 
ROWID		      CUST_ID
------------------ ----------
AAAR4qAAMAAAAedAAA	  101
 
SQL> select * from table(dbms_xplan.display_cursor(format=>'allstats last'));
 
PLAN_TABLE_OUTPUT
-----------------------------------------
SQL_ID	9g2nyr9h2ytk4, child number 0
-------------------------------------
select /*+ gather_plan_statistics */ rowid,cust_id from DEMO where
cust_id=101
 
Plan hash value: 3286081706
 
------------------------------------------------------------------------------------
| Id  | Operation	  | Name | Starts | E-Rows | A-Rows |	A-Time	 | Buffers |
------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |	 |	1 |	   |	  1 |00:00:00.01 |	 1 |
|*  1 |  TABLE ACCESS HASH| DEMO |	1 |	 1 |	  1 |00:00:00.01 |	 1 |
------------------------------------------------------------------------------------
 
Predicate Information (identified by operation id):
---------------------------------------------------
 
   1 - access("CUST_ID"=101)

Due to the prefetch of 2 rows Oracle detects that there actually is only 1 row and avoids the second logical IO (a second fetch).
If cust_id is unique then I would have created a unique (or primary) key constraint here, which would avoid a second fetch as well (because Oracle knows from the constraint that there can be max 1 row per cust_id), but in that case I have to maintain the created index.

I made a couple of tests, which compared the behaviour with different settings of rowprefetch and arraysize in sqlplus (what is actually the difference between the 2 settings?). That will be a subject of a future Blog.

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Clemens Bleile
Clemens Bleile

Technology Leader & Principal Consultant