Oracle SQL Tuning

Presented ByAkin S Walter-Johnson Ms

Principal PeerLabs, Inc

http://www.peerlabs.com

Email akin@peerlabs.com

SCOPE

• How data is accessed and reconstituted joins

• Inform the user on how identify problems with SQL

• Repair of SQL

• Tuning can occur at 2 levels– Server ( DBA)– SQL level ( User)

IMPORTANCE OF TUNING

• Reduce response time for SQL processing

• To find a more efficient way to process workload

• Improve search time by using indexes

• Join data efficiently between 2 or more tables

HOW TO TUNE

• Review the access path, Join methods and index usage

• Test response through SQPLUS directly ( May mask performance )

• Test response through an Application front end ( Usually takes longer )

• Test response through a web interface

ROLE OF HARDWARE & DESIGN

• All the hardware in world will not save you• Memory, Disk & CPU speed can improve

performance• Increased hardware does not always result into

better performance• Poor application design accounts for over 70%

of performance issues• Do Performance design review early in

development

OVERVIEW OF SQL PROCESSING

OVERVIEW OF SQL PROCESSING

• The Parser checks both syntax and semantic analysis of SQL statement

• Optimizer determines the most efficient way of producing the result of the query also known as the EXPLAIN PLAN. How best to get the data.

• Oracle Optimizer types ( Cost Based and Rule Based )– CBO based Optimizer uses cost associated with each execution

requires you to analyze objects for statistics– RULE based Optimizer internal rules ( not encouraged by oracle)

• The SQL Execution Engine operates on the execution plan associated with a SQL statement and then produces the results of the query.

SETTING OPTIMIZER

• SERVER Level by DBA in parameter file (init.ora)• CLIENT Level SQLPLUS command < alter session set

optimizer_mode=choose>• STATEMENT Level using hints

– a. select /*+RULE */ * from dual ;– b. select /*+ CHOOSE */ * from dual ;

• Order of Precedence• SERVER->CLIENT->STATEMENT• Users can set both client and statement• To use CBO you need to analyze the tables (see

Analyze objects)

OPTIMIZER OPERATIONS THAT AFFECT PERFORMANCE

• The Optimizer is the brain behind the process of returning data to user it needs to make the following choices.

• OPTIMIZER APPROACH• ACCESS PATH• JOIN ORDER• JOIN METHOD• Choice of optimizer approaches• CBO or RULE• Choice of Access Paths ( How data is Scanned )• Use an index if not reading all records ( faster)• Read or scan all records• Choice of Join Orders• Determine which table to join first when you have more than two tables in

an SQL• Choice of Join Methods• Determine how to join the tables ( Merge, Sort, Hash )

SQLPLUS ENVIRONMENT LAB

• Log on

• Set timing

• Auto Trace to see plan ( How SQL is processed )

• Set optimizer

• Review Plan

ANALYZE OBJECT STATISTICS

• Statistics describe physical attributes of an object such as– Number of rows, average space, empty blocks

• All objects need to have statistics to use CBO• Stored in user_tables and user_indexes• Not update automatically use analyze• Table Statistics

– Table Name – Number of rows– Average space– Total number of blocks– Empty blocks

• Index Statistics– Index_Name– Index_Type– Table_Name – Distinct_Keys– Avg_Leaf_Blocks_Per_Key – Avg_Data_Blocks_Per_Key

ANALYZE OBJECT STATISTICS LAB

• Create Table

• Create Index

• Review tables

• Review indexes

TABLE TUNING (i)• A Table in oracle store data• Resides in a schema within a Table-space• Contains actual data stored in oracle blocks• An oracle block is a multiple of the OS block (Ask your DBA)• Row Chaining (Performance killer)

– A row is too large to fit into on data block so oracle uses more than one chaining them

– Chaining occurs when you try to inset or update• Row migration (Performance killer)

– There is not enough place in the BLOCK for UPDATES– Oracle tries to find another Block with enough free space to hold the entire row.

( Unnecessary scanning)– If a free block is available Oracle moves the entire ROW to the NEW BLOCK.– Oracle keeps the original Row piece of a Migrated row row to POINT to the NEW

BLOCK• Queries that select from chained or migrated rows must perform double read and

write (I/O.• To find Chained or Migrated table run

– SQL> ANALYZE TABLE SCHEMA_NAME.TABLE_NAME LIST CHAINED ROWS;

– SQL> select CHAIN_CNT from user_tables ;

TABLE TUNING (ii)

• Too many empty blocks

• Occurs after a massive delete then inserting few records

• Select statement takes a very long time with only one record in table

• Solution is to TRUNCATE the table and copy to new table

TABLE TUNNING LAB

WHY USE AN INDEX

• What is an Index– A pointer or a hand that directs to something– Similar to index at the end of a book

• Oracle Index– Binary tree Structure with entries know as ROWID– Left nodes contain key and rowid– ROWID is internal and points to direct location of

record on disk– ROWID is fasted way to reach a record.– SQL> Select rowid, id, name from mytable ;

OPTIMIZER ACCESS by ROWID

• ROWID SCAN– The fastest way to get a row– Based on the file and the data block where

record is located– Used also during an index scan

OPTIMIZER ACCESS by INDEX UNIQUE SCAN

• The scan returns only one row• It requires an index (Primary key)on the Table• Index is automatically created for primary key• Used by Optimizer

– When an index exist on a column with a where clause– When the optimizer is told to use an index (hint) Index

hints are not really used.

• Reading Explain Plan – Do a unique scan of the index and obtain ROWID– Access the table by ROWID

OPTIMIZER ACCESS by INDEX RANGE SCAN

• The scan may return more than one row

• Used by optimizer when – where clause has > or < sign– where clause has between 10 and 20– where clause has like * ( wild card)

OPTIMIZER ACCESS by MULTIPLE UNIQUE SCAN

• Optimizer will search for ROWID in the statement

• Concatenate all records into one row set– Combining all rows selected by the unique

scan into I row set

• Used by Optimizer when– where clause has an in condition id IN ( 123,

456, 678 )

OPTIMIZER ACCESS by MULTIPLE UNIQUE SCAN

• Multiple Unique ScanIndex unique scan 123

Index unique scan 456

Index unique scan 456

Table access byrowid

Table access byrowid

Table access byrowid

Concatenation

OPTIMIZER ACCESS by FULL TABLE SCAN

• Each record is read one by one

• A sequential search for data no index is used

• The slowest search

• Occurs when – There is no index or index disabled– When the Optimizer is hinted not to use the

index

OPTIMIZER ACCESS by FAST FULL INDEX SCAN

• Alternative to a full table scan• Used by optimizer when

– Index contains all the columns that are needed for the query

– If I want to display only your SSN, you don’t have to access the table if I have SSN as an index

• A fast full scan accesses the data in the index itself, without accessing the table

OPTIMIZER JOIN METHOD

• A query with more than one table requires to have a Join Order

• Join Order are steps taken to assemble rows of data from more than one table

• Select From A,B,C Where A.col1 = B.Col1 And B.Col2 = C.Col2

• NESTED LOOP• SORT-MERGE• HASH JOIN

OPTIMIZER JOIN METHOD NESTED LOOP

• Nested Loop

• Uses a Looping method to join 2 table

• For every record in A we look thru all rows in B using an index to find a match

• Table A is Outer Loop or Driving table

• Table B is Inner Loop

Nested Loop

Table A (OuterLoop or Driving

Table B ( Inner Loop )

Loop Thru

0 SELECT STATEMENT Optimizer=CHOOSE 1 0 NESTED LOOPS 2 1 TABLE ACCESS (FULL) OF 'EMP' 3 1 INDEX (UNIQUE SCAN) OF 'PK_DEPTNO' (UNIQUE)

Nested Loop

• Good when you expect a small number of rows back• Good for Small driving table so not Good if driving table

is large• Good when Index on B exist ( will perform poor when no

index on B )• Good if you want to quickly returns data to the screen

( ONLINE USERS )• HINT select • SELECT /*+ ORDERED USE_NL(DEPT) to get first row

faster */• EMPNO, ENAME, DEPT.DEPTNO• FROM EMP, DEPT • WHERE EMP.DEPTNO = DEPT.DEPTNO ;

OPTIMIZER JOIN METHOD SORT MERGE JOIN

• In a merge join, there is no concept of a driving table

• The join consists of two steps:

• Sort join operation: Both the inputs are sorted on the join key.

• Merge join operation: The sorted lists are merged together.

OPTIMIZER JOIN METHOD SORT MERGE JOIN

• Merge JoinFULL TABLE ScanTable A

FULL TABLE ScanTable B

SORTTable A

SORTTable A

MERGEA & B

SELECT STATEMENT Optimizer=CHOOSE1 0 MERGE JOIN2 1 SORT (JOIN)3 2 TABLE ACCESS (FULL) OF 'DEPT'4 1 SORT (JOIN)5 4 TABLE ACCESS (FULL) OF 'EMP'

OPTIMIZER JOIN METHOD SORT MERGE JOIN

• The Merge can’t begin until data sorted from both tables• Since there is a waiting period, this join method will not

be good for ONLINE users• Good when you don’t have an index on the join columns,

if Index exist a NESTED LOOP is done• Good when NESTED LOOP does not perform when• Good if rows are loaded in a sorted fashion• Not Good if you want to quickly return data to the screen

( ONLINE USERS ) need to wait for sorting.• Not Good is one of the tables is very,very large because

a Full table scan will be done.• Good if working with a oracle parallel options because

SORTING can be done in parallel

OPTIMIZER JOIN METHOD SORT MERGE JOIN

• Optimizer will use SORT JOIN when index does not exist (May be a warning)

• Optimizer will use SORT JOIN when OPTIMIZER_MODE is Rule

• Optimizer will use SORT JOIN when HASH_JOIN_ENABLED is false.

• HINT

• SELECT /*+ USE_NL(l h) */

OPTIMIZER JOIN METHOD HASH JOIN

• HASH join compares tow tables in memory to find matching rows

• Must set HASH_JOIN_ENABLED to True(DBA )• Read first table into memory via Full table scan• Apply hashing function to data to prepare for join

on key fields• Read second table via Full table scan • Apply hashing function to compare the second to

the first table

OPTIMIZER JOIN METHOD HASH JOIN

• HASH JOIN

FULL TABLE ScanTable A

FULL TABLE ScanTable B

HASH JOINA & B

0 SELECT STATEMENT Optimizer=CHOOSE (Cost=5 Card=82 Bytes=3772 ) 1 0 HASH JOIN (Cost=5 Card=82 Bytes=3772) 2 1 TABLE ACCESS (FULL) OF 'EMP' (Cost=2 Card=82 Bytes=2706) 3 1 TABLE ACCESS (FULL) OF 'DEPT' (Cost=2 Card=82 Bytes=1066)

OPTIMIZER JOIN METHOD HASH JOIN

• Good only when you have parallel options for oracle because of FTS

• Good if you have more memory set aside for hashing functions

• Good if you indexes don't perform well with NESTED LOOP

• May be faster than NESTED LOOP because you are reading in memory as supposed to using index

• Better than SORT MERGE because only on table has to be sorted

OPTIMIZER JOIN METHOD HASH JOIN

• HINT

• SELECT /*+use_hash(emp, dept )*/

• EMPNO, ENAME, DEPT.DEPTNO

• FROM EMP, DEPT

• WHERE EMP.DEPTNO = DEPT.DEPTNO ;

OPTIMIZER JOIN ORDER

• Avoid performing unnecessary work to access rows that do not affect the result.

• Choose the best join order, driving to the best unused filters earliest.

OPTIMIZER JOIN ORDER

• Query 1• SELECT info• FROM taba a, tabb b, tabc c• WHERE • a.key1 = b.key1• AND a.key2 = c.key2• AND a.acol BETWEEN 100 AND 200• AND b.bcol BETWEEN 10000 AND 20000• AND c.ccol BETWEEN 10000 AND 20000

OPTIMIZER JOIN ORDER

• Query 2 • SELECT info• FROM taba a, tabb b, tabc c• WHERE • a.acol BETWEEN 100 AND 200• AND b.bcol BETWEEN 10000 AND 20000• AND c.ccol BETWEEN 10000 AND 20000• AND a.key1 = b.key1• AND a.key2 = c.key2;

OPTIMIZER JOIN ORDER

• Query3• SELECT info• FROM taba a, tabb b, tabc c• WHERE • b.bcol BETWEEN 10000 AND 20000• AND c.ccol BETWEEN 10000 AND 20000• AND a.acol BETWEEN 100 AND 200• AND a.key1 = b.key1• AND a.key2 = c.key2;

OPTIMIZER JOIN ORDER

• The work of the following join can be reduced by first joining to the table with the best still-unused filter.

• Thus, if "bcol BETWEEN ..." is more restrictive (rejects a higher percentage of the rows seen) than "ccol BETWEEN ...", the last join can be made easier (with fewer rows) if tabb is joined before tabc.

OPTIMIZER JOIN ORDER

• The driving table is the one containing the filter condition that eliminates the highest percentage of the table.

• Thus, because the range of 100 to 200 is narrow compared with the range of acol,

• but the ranges of 10000 and 20000 are relatively large, taba is the driving table, all else being equal.

• HINT• The ORDERED hint causes Oracle to join tables in

the order in which they appear in the FROM clause.

INDEX TUNING

• Rebuild you index often ( index oil change)

• Gather statistics

• Do not over use Indexes

• Restrict to cols that return a few records

• Use Bitmapped index when number of values is small e.g ( Sex male, female)

• Suppression of index • Select * from mytable where total + 3 = 20

REPAIR YOUR SQL STATEMENT LAB

• Understand the purpose of a statement re-writing may improve performance.

• Use equi joins on where clause• Avoid column transformation

– Where to_number(a.id) = b.id– Do not use function in predicate– Where to_string(a.id) = b.id– col1 = NVL (:b1,col1) – NVL (col1,-999) = .... – TO_DATE(), TO_NUMBER(), and so on

REPAIR YOUR SQL STATEMENT LAB

• WRITE SEPARATE SQL STATEMENTS FOR SPECIFIC TASKS

• It is better to use IN rather than EXISTS. , if the selective predicate is in the subquery, then use IN. If the selective predicate is in the parent query, then use EXISTS.

SQL TUNING CHECK LIST

• Ask DBA if Cost based optimizer is default in init.ora

• Check if you have statistics for tables and indexes

• Check if you have a high number of empty blocks on tables(due to large deletes)

• Check if you have row chaining or row migration on Tables

• Check index cluster• Review SQL plan• Use more packages and stored procedures