Introduction to SQL Tuning Brown Bag Three essential concepts.

Introduction to SQL TuningBrown Bag

Three essential concepts

Introduction to SQL Tuning

•How to speed up a slow query?

• Find a better way to run the query

• Cause the database to run the query your way

Introduction to SQL Tuning

•How does a database run a SQL query?

• Join order

• Join method

• Access method

Example Query

SQL> select 2 sale_date, product_name, customer_name, amount 3 from sales, products, customers 4 where 5 sales.product_number=products.product_number and 6 sales.customer_number=customers.customer_number and 7 sale_date between 8 to_date('01/01/2012','MM/DD/YYYY') and 9 to_date('01/31/2012','MM/DD/YYYY') and 10 product_type = 'Cheese' and 11 customer_state = 'FL';

SALE_DATE PRODUCT_NAME CUSTOMER_NAME AMOUNT--------- ------------ ----------------- ----------04-JAN-12 Feta Sunshine State Co 30002-JAN-12 Chedder Sunshine State Co 10005-JAN-12 Feta Green Valley Inc 40003-JAN-12 Chedder Green Valley Inc 200

Join Order

•Join Order = order in which tables in from clause are joined

•Two row sources at a time

•Row source:

•Table

•Result of join

•View as tree – execution tree or plan

Join Order – sales, products, customers

products sales

join 1 customers

join 2

Join Order as Plan

Execution Plan---------------------------------------------------------- 0 SELECT STATEMENT 1 0 HASH JOIN 2 1 HASH JOIN 3 2 TABLE ACCESS (FULL) OF 'SALES' (TABLE) 4 2 TABLE ACCESS (FULL) OF 'PRODUCTS' (TABLE 5 1 TABLE ACCESS (FULL) OF 'CUSTOMERS' (TABLE)

Bad Join Order – customers, products, sales

productscustomers

join 1 sales

join 2

Cartesian Join – all products to all customers

SQL> -- joining products and customersSQL> -- cartesian joinSQL> SQL> select 2 product_name,customer_name 3 from products, customers 4 where 5 product_type = 'Cheese' and 6 customer_state = 'FL';

PRODUCT_NAME CUSTOMER_NAME------------ -----------------Chedder Sunshine State CoChedder Green Valley IncFeta Sunshine State CoFeta Green Valley Inc

Plan with Cartesian Join

Execution Plan---------------------------------------------------------- 0 SELECT STATEMENT Optimizer=ALL_ROWS 1 0 MERGE JOIN (CARTESIAN) 2 1 TABLE ACCESS (FULL) OF 'PRODUCTS' (TABLE) 3 1 BUFFER (SORT) 4 3 TABLE ACCESS (FULL) OF 'CUSTOMERS' (TABLE)

Selectivity

•Selectivity = percentage of rows accessed versus total rows

•Use non-joining where clause predicates

•sale_date, product_type, customer_state

•Compare count of rows with and without non-joining predicates

Count(*) to get selectivity

-- # selected rows

select count(*) from saleswheresale_date between to_date('01/01/2012','MM/DD/YYYY') and to_date('01/31/2012','MM/DD/YYYY'); -- total #rows select count(*) from sales;

Selectivity of sub-tree

SQL> select count(*) from sales, products 3 where 4 sales.product_number=products.product_number and 5 sale_date between 6 to_date('01/01/2012','MM/DD/YYYY') and 7 to_date('01/31/2012','MM/DD/YYYY') and 8 product_type = 'Cheese';

COUNT(*)---------- 4

SQL> select count(*) 2 from sales, products 3 where 4 sales.product_number=products.product_number;

COUNT(*)---------- 4

Modifying the Join Order

•Tables with selective predicates first

•Gather Optimizer Statistics

•Estimate Percent

•Histogram on Column

•Cardinality Hint

•Leading Hint

•Break Query into Pieces

Gather Optimizer Statistics

-- 1 - set preferences

begin

DBMS_STATS.SET_TABLE_PREFS(NULL,'SALES','ESTIMATE_PERCENT','10');DBMS_STATS.SET_TABLE_PREFS(NULL,'SALES','METHOD_OPT', 'FOR COLUMNS SALE_DATE SIZE 254 PRODUCT_NUMBER SIZE 1 '|| 'CUSTOMER_NUMBER SIZE 1 AMOUNT SIZE 1');

end;/

-- 2 - regather table stats with new preferences

execute DBMS_STATS.GATHER_TABLE_STATS (NULL,'SALES');

Cardinality Hint

SQL> select /*+cardinality(sales 1) */ 2 sale_date, product_name, customer_name, amount 3 from sales, products, customers 4 where 5 sales.product_number=products.product_number and 6 sales.customer_number=customers.customer_number and 7 sale_date between 8 to_date('01/01/2012','MM/DD/YYYY') and 9 to_date('01/31/2012','MM/DD/YYYY') and 10 product_type = 'Cheese' and 11 customer_state = 'FL';


Plan with Cardinality hint

Execution Plan---------------------------------------------------------- 0 SELECT STATEMENT Optimizer=ALL_ROWS 1 0 HASH JOIN 2 1 HASH JOIN 3 2 TABLE ACCESS (FULL) OF 'SALES' (TABLE) 4 2 TABLE ACCESS (FULL) OF 'PRODUCTS' (TABLE 5 1 TABLE ACCESS (FULL) OF 'CUSTOMERS' (TABLE)

Leading Hint

SQL> select /*+leading(sales) */ 2 sale_date, product_name, customer_name, amount 3 from sales, products, customers 4 where 5 sales.product_number=products.product_number and 6 sales.customer_number=customers.customer_number and 7 sale_date between 8 to_date('01/01/2012','MM/DD/YYYY') and 9 to_date('01/31/2012','MM/DD/YYYY') and 10 product_type = 'Cheese' and 11 customer_state = 'FL';


Break Query Into Pieces

SQL> create global temporary table sales_product_results 2 ( 3 sale_date date, 4 customer_number number, 5 amount number, 6 product_type varchar2(12), 7 product_name varchar2(12) 8 ) on commit preserve rows;

Table created.


SQL> insert /*+append */ 2 into sales_product_results 3 select 4 sale_date, 5 customer_number, 6 amount, 7 product_type, 8 product_name 9 from sales, products 10 where 11 sales.product_number=products.product_number and 12 sale_date between 13 to_date('01/01/2012','MM/DD/YYYY') and 14 to_date('01/31/2012','MM/DD/YYYY') and 15 product_type = 'Cheese';

4 rows created.


SQL> select 2 sale_date, product_name, customer_name, amount 3 from sales_product_results spr, customers c 4 where 5 spr.customer_number=c.customer_number and 6 c.customer_state = 'FL';

SALE_DATE PRODUCT_NAME CUSTOMER_NAME AMOUNT--------- ------------ ----------------- ----------02-JAN-12 Chedder Sunshine State Co 10003-JAN-12 Chedder Green Valley Inc 20004-JAN-12 Feta Sunshine State Co 30005-JAN-12 Feta Green Valley Inc 400

Join Methods

•Join Method = way that data from two sources is joined

•Nested Loops

•Small number of rows in first table

•Unique index on second large table

•Hash Join

•Smaller or equal number of rows in first table

•No index required

Join Method – Nested Loops

Execution Plan------------------------------------------------------------------ 0 SELECT STATEMENT Optimizer=ALL_ROWS 1 0 TABLE ACCESS (BY INDEX ROWID) OF 'CUSTOMERS' (TABLE) 2 1 NESTED LOOPS 3 2 NESTED LOOPS 4 3 TABLE ACCESS (FULL) OF 'SALES' (TABLE) 5 3 TABLE ACCESS (BY INDEX ROWID) OF 'PRODUCTS' 6 5 INDEX (RANGE SCAN) OF 'PRODUCTS_INDEX' (INDEX) 7 2 INDEX (RANGE SCAN) OF 'CUSTOMERS_INDEX' (INDEX)

Join Method – Hash Join

Execution Plan---------------------------------------------------------- 0 SELECT STATEMENT Optimizer=ALL_ROWS 1 0 HASH JOIN 2 1 HASH JOIN 3 2 TABLE ACCESS (FULL) OF 'SALES' (TABLE) 4 2 TABLE ACCESS (FULL) OF 'PRODUCTS' 5 1 TABLE ACCESS (FULL) OF 'CUSTOMERS' (TABLE)

Modifying the Join Method

•Hints

•use_hash

•use_nl

•Add Index

•Hash_area_size parameter

Join Methods Hints

/*+ use_hash(products) use_nl(customers) */

Join Methods Indexes

create index products_index on products(product_number);

create index customers_index on customers(customer_number);

Join Methods Hash_Area_Size

NAME TYPE VALUE------------------------------------ ----------- ---------hash_area_size integer 100000000sort_area_size integer 100000000workarea_size_policy string MANUAL

Access Methods

•Access method = way that data is retrieved from table

•Index scan – small number of rows accessed

•Full scan – larger number of rows accessed

Modifying the Access Method

•Set Initialization Parameter

•optimizer_index_caching

•optimizer_index_cost_adj

•db_file_multiblock_read_count

•Set Parallel Degree > 1

•Hints

•Full

•Index

Set Initialization Parameter

alter system set optimizer_index_cost_adj=1000 scope=both sid='*';

Set Parallel Degree

alter table sales parallel 8;

Full Scan and Index Hints

/*+ full(sales) index(customers) index(products) */

Conclusion

• Use count queries to determine selective parts of where clause

• Modify the join order, join methods, and access methods using

• Optimizer statistics

• Hints

• Initialization parameters

• Breaking the query into pieces

• Parallel degree

• Indexes

• Compare elapsed time of query with new plan to original

Check For Improved Elapsed Time

SQL> set timing onSQL> SQL> select …

… removed for clarity …

SALE_DATE PRODUCT_NAME CUSTOMER_NAME AMOUNT--------- ------------ ----------------- ----------02-JAN-12 Chedder Sunshine State Co 10003-JAN-12 Chedder Green Valley Inc 20004-JAN-12 Feta Sunshine State Co 30005-JAN-12 Feta Green Valley Inc 400

Elapsed: 00:00:00.00

Further Reading

•Oracle Database Concepts

•Chapter 7 SQL

•Oracle Database Performance Tuning Guide

•Chapter 11 The Query Optimizer

•Chapter 19 Using Optimizer Hints

•Oracle Database Reference

•Chapter 1 Initialization Parameters

•Oracle Database PL/SQL Packages and Types Reference

•Chapter 141 DBMS_STATS

•Cost-Based Oracle Fundamentals - Jonathan Lewis

•http://www.bobbydurrettdba.com/resources/

Introduction to SQL Tuning Brown Bag Three essential concepts.

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