SQL Tuning Guideline

Sidney · 2011 Feb 16

Agenda

• The Guideline to tune sql1. Application Logic

2. Table/Column statistics

3. SQL access path

4. SQL join NL or hash Join

Response Time

• The core of user experience

Why? So Slow!

1. Application Logic

2. Tune at sql level

How to review sql

• Verifying Optimizer Statistics

• Reviewing the Execution Plan

• Restructuring the SQL Statements(code change)

• Restructuring the Indexes

SQLT: Nice tool review top sql

• Statistics:Tables/Indexes/Columns

• Execution plan1. Predication

2. Bind value

3. SQL profile

Table/Index stats

• Table stats1. Last_Analyzed2. Num_Rows3. Sample_Size

• Index stats1. Last_Analyzed2. Num_Rows3. Sample_Size4. DISTINCT_KEYS5. BLEVEL6. Clustering_Factor

Column statistics

• num_rows

• num_distinct• density• num_nulls• num_buckets• histogram• low_value• high_value

How to gather Table/Column statistics

• Using auto method– exec

dbms_stats.gather_table_stats('CS2_PARTY_OWNER','CS2_BKG_RQST',method_opt=>'for all columns size auto',cascade=>true);

• For specific columns– exec

dbms_stats.gather_table_stats('CS2_PARTY_OWNER','CS2_BKG_RQST',method_opt=>'for columns size 254 SP_COMPANY_ID',cascade=>true);

Column Histogram

Review execution plan

• Driving table has the best filter

• Join method: Nested Loop or Hash Join

• Join order: Fewest number of rows are being returned to the next step

• Each table is being accessed efficiently1. index scan2. full table scan

Nested LoopFor small number of rows(<1000), with a good driving condition

between the two tables

Hash Function

Hash Join For large data set(>1000): The optimizer uses the smaller of two tables or data sources

to build a hash table on the join key in memory

Access Path

• Full Table Scans

• Index Scans1. Index Unique Scans2. Index Range Scans3. Full Scans4. Fast Full Index Scans5. Index Joins

SQL Profile

• SQL Profiles are the new feature from 10g which is managed by Oracle Enterprise Manager as part of the Automatic SQL Tuning process. Apart from OEM, SQL Profiles can be managed through the DBMS_SQLTUNE package.

normal usage of SQL Profile

• Run the SQL Tuning Advisor

• Accept the recommended SQL Profile

advance usage:coe_xfr_sql_profile.sql

• coe_xfr_sql_profile.sql is from SQLT tool

• An easy way to lock the sql plan as outline

Output sql format

• coe_xfr_sql_profile_<SID>_<PLAN_Hash_value>.SQL

• coe_xfr_sql_profile_chpub18xajb2w_1020905707.SQL

Q&A