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Session Objectives And Key Takeaways Session Objectives: Dive into the query performance and tuning capabilities in SQL Server Demonstrate common tuning and troubleshooting techniques using SQL Server Disclaimer: Query Tuning is a deep & broad topic – won’t cover everything Key Takeaways: SQL Server has wealth of capabilities Get a refresher on certain fundamental concepts Common pitfalls to avoid and optimization techniques

Factors Affecting Performance? Application Design 3 tier architecture facilitates caching, connection pooling. Partitioning tables and databases Not mixing logs and database on the same disk Database Design Normalization of database structure, choosing indexes. Microsoft SQL Server Setup. Affinity mask, lightweight pooling, max worker threads and degree of parallelism. Hardware

Indexes Scan and Seek * Clustered and non-clustered indexes Seekable Predicates * Single column Indexes Multi Column Indexes Covered Columns A Clustered index covers all columns A non-clustered index covers only key columns A non-clustered index on a table with a clustered index covers the clustered index key

Example : Covered Columns CREATE TABLE T_heap (a int, b int, c int, d int, e int, f int) CREATE INDEX T_heap_a ON T_heap (a) CREATE INDEX T_heap_bc ON T_heap (b, c) CREATE INDEX T_heap_d ON T_heap (d) INCLUDE (e) CREATE UNIQUE INDEX T_heap_f ON T_heap (f) CREATE TABLE T_clu (a int, b int, c int, d int, e int, f int) CREATE UNIQUE CLUSTERED INDEX T_clu_a ON T_clu (a) CREATE INDEX T_clu_b ON T_clu (b) CREATE INDEX T_clu_ac ON T_clu (a, c) CREATE INDEX T_clu_d ON T_clu (d) INCLUDE (e) CREATE UNIQUE INDEX T_clu_f ON T_clu (f)

Indexing Bookmark Lookups * Bookmark lookups are expensive Index Cost * Only one clustered index allowed per table A Clustered index is always unique even if it is not explicitly specified so. Create a PRIMARY KEY by adding a NONCLUSTERED keyword. Make non-clustered indexes highly selectable.

Indexing – cont’d Index small fields. Avoid creating indexes on large string columns ALTER TABLE titles ADD hash_title AS CHECKSUM(title) CREATE INDEX hash_index ON titles(hash_title) SELECT * FROM titles WHERE title = 'Cooking with Computers: Surreptitious Balance Sheets' AND hash_title = CHECKSUM('Cooking with Computers: Surreptitious Balance Sheets')

Indexing – cont’d Pay attention to column order Drop or create indexes as needed

Plan Caching Adhoc plan caching * Autoparameterization * Forced Parameterization ALTER DATABASE SET PARAMETERIZATION FORCED; Use Prepared statements *

Stored Procedures Execution Plan Usage Keep them small Recompiles Causes Avoiding Analysis

Stored Procedure Recompile recommendations Avoid interleaving DDL and DML operations Fully qualify all references to SP Use KEEP PLAN/KEEP FIXEDPLAN option To avoid recompile, execute statements by using either of these Sub-procedures EXECUTE/sp_executesql Use table variable instead of temp table (New in Microsoft SQL Server 2000)

Transactions Keep them short Avoid external work inside the transaction such as Take care to avoid deadlocks Use a appropriate isolation level.

Best Practices LEARN T-SQL Avoid Cursors as far as possible Do you have missing indexes – Take the help of DTA Avoid joining tables on different data types For frequent computations create a calculated column It helps in column statistics which SQL Server can use in optimizing the query. Don’t use unqualified object names

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