Query Optimization

Comprehensive guide to T-SQL query optimization for SQL Server and Azure SQL Database. Optimize from verified evidence: schema, data types, indexes, row counts, partitioning, execution plans, and allowed change types.

Mandatory Workflow

1. Schema-First Validation

Before recommending rewrites, indexes, or hints, verify or mark unknown:

• SQL Server version, edition, compatibility level, and Azure SQL tier if applicable.
• Full query/procedure text, parameter values, frequency, and runtime target.
• DDL for tables, views, temp tables, table variables, and TVFs.
• Data types for join, filter, grouping, ordering, and temp-table columns, including length, precision, scale, collation, and nullability.
• Existing indexes, constraints, statistics age, partition function/scheme, and row counts.
• Whether objects are local or linked-server/remote sources.
• Allowed change types: query rewrite, new index, huge-table index change, stats update, staging table, computed column, or no code change.

Use ../_shared/optimization-intake.md and ../_shared/assumption-tracker.md. If key facts are missing, provide conditional guidance plus diagnostics instead of final prescriptions.