𝗙𝗶𝘅 𝗦𝗹𝗼𝘄 𝗗𝗮𝘁𝗮𝗯𝗮𝘀𝗲 𝗤𝘂𝗲𝗿𝗶𝗲𝘀

𝗙𝗶𝘅 𝗦𝗹𝗼𝘄 𝗗𝗮𝘁𝗮𝗯𝗮𝘀𝗲 𝗤𝘂𝗲𝗿𝗶𝗲𝘀

Slow queries break your app. One bad query ruins the experience for all users. You must learn to read query plans.

Find slow queries first.

• Use pg_stat_statements for PostgreSQL.
• Use slow query logs for MySQL.
• Use sys.dm_exec_query_stats for SQL Server.

Use EXPLAIN ANALYZE to see execution.

• Look for sequential scans on large tables. This means you need indexes.
• Look for nested loop joins with too many rows.

Add the right indexes.

• Index columns in WHERE clauses.
• Index JOIN and ORDER BY columns.
• Use composite indexes for multiple filters.
• Put equality filters first.
• Put range filters last.

Use covering indexes. These hold all columns for a query. The database skips the table. This is an index-only scan. It is fast.

Rewrite your queries.

• Replace OR with UNION ALL.
• Use EXISTS instead of IN for subqueries.
• Avoid functions in WHERE clauses.

Partition huge tables.

• Partition by date for time data.
• Partition by region for location data.
• The database scans fewer partitions.

Test with production volumes. Dev data is too small. Use anonymous production data. Measure speed before and after you deploy.

Source: https://dev.to/therizwansaleem/database-query-optimization-reading-query-plans-and-fixing-slow-queries-5pk Optional learning community: https://t.me/GyaanSetuAi