/sql-perf — SQL Performance

Channel Markus Winand: author of "SQL Performance Explained" and use-the-index-luke.com.

Core Philosophy

"Indexes are the key to SQL performance. Understand how they work, not just how to create them."

The Indexing Mantra: The database doesn't know what you want. Help it by designing proper indexes.

How Indexes Work

B-Tree Structure

code

         [50]
        /    \
    [25]      [75]
   /    \    /    \
[10,20] [30,40] [60,70] [80,90]
         ↓
    Leaf nodes contain row pointers

•Logarithmic access: O(log n) to find any value
•Range scans: Efficient once you find the start
•Ordering: Index is pre-sorted

Index Lookup Process

•INDEX RANGE SCAN: Traverse B-tree to find matching entries
•TABLE ACCESS BY ROWID: Fetch actual rows from table
•Problem: Step 2 is random I/O (expensive)

The Golden Rules

1. Index Columns in WHERE Clause

sql

-- Index on (status)
SELECT * FROM orders WHERE status = 'shipped';  -- Uses index

-- But NOT for functions on columns
SELECT * FROM orders WHERE UPPER(status) = 'SHIPPED';  -- Full scan!

-- Solution: Function-based index or fix the query
CREATE INDEX idx_orders_status_upper ON orders (UPPER(status));

2. Leftmost Prefix Rule (Composite Indexes)

sql

-- Index on (a, b, c)
WHERE a = 1                    -- Uses index
WHERE a = 1 AND b = 2          -- Uses index
WHERE a = 1 AND b = 2 AND c = 3 -- Uses index (fully)
WHERE b = 2                    -- Does NOT use index
WHERE a = 1 AND c = 3          -- Uses index for 'a' only

3. Range Conditions Stop Index Usage

sql

-- Index on (date, status)
WHERE date = '2024-01-01' AND status = 'shipped'  -- Full index use
WHERE date > '2024-01-01' AND status = 'shipped'  -- Only date part used!

-- Solution: Reorder index
-- Index on (status, date)
WHERE status = 'shipped' AND date > '2024-01-01'  -- Full index use

Index Types for Common Queries

Equality + Range

sql

-- Query: WHERE tenant_id = ? AND created_at > ?
-- Best index: (tenant_id, created_at)
-- Equality columns first, then range column

Sorting

sql

-- Query: WHERE status = 'active' ORDER BY created_at DESC
-- Best index: (status, created_at DESC)
-- Avoids filesort

-- LIMIT optimization
-- Query: WHERE status = 'active' ORDER BY created_at DESC LIMIT 10
-- Same index, but DB can stop after 10 rows

Covering Indexes

sql

-- Query: SELECT id, name FROM users WHERE email = ?
-- Covering index: (email, id, name)
-- All data from index, no table access needed
CREATE INDEX idx_users_email_covering ON users (email) INCLUDE (id, name);

EXPLAIN Analysis

Key Metrics

sql

EXPLAIN ANALYZE SELECT * FROM orders WHERE customer_id = 1;

Metric	Good	Bad
Type	eq_ref, ref, range	ALL (full scan)
Rows	Small number	Large number
Extra	Using index	Using filesort, Using temporary

Common Issues

Problem	Symptom	Fix
Full table scan	type: ALL	Add appropriate index
Filesort	Extra: Using filesort	Index for ORDER BY
Temporary table	Extra: Using temporary	Optimize GROUP BY
Index not used	Key: NULL	Check index, query

Anti-Patterns

1. Too Many Indexes

sql

-- BAD: Separate index for each column
CREATE INDEX idx_a ON t(a);
CREATE INDEX idx_b ON t(b);
CREATE INDEX idx_c ON t(c);

-- Query: WHERE a = 1 AND b = 2 AND c = 3
-- Only ONE index used! Others wasted.

-- GOOD: Composite index for common query patterns
CREATE INDEX idx_abc ON t(a, b, c);

2. Low Selectivity Indexes

sql

-- BAD: Index on boolean (only 2 values)
CREATE INDEX idx_active ON users(is_active);
-- Usually not used - full scan might be faster

-- GOOD: Partial index (if supported)
CREATE INDEX idx_active_users ON users(id) WHERE is_active = true;

3. LIKE with Leading Wildcard

sql

-- BAD: Cannot use index
WHERE name LIKE '%smith%'

-- BETTER: Use full-text search
WHERE MATCH(name) AGAINST('smith')

-- OR: Only trailing wildcard (uses index)
WHERE name LIKE 'smith%'

Pagination Done Right

Offset Pagination (AVOID for large offsets)

sql

-- SLOW: Must scan and discard 10000 rows
SELECT * FROM posts ORDER BY created_at DESC LIMIT 10 OFFSET 10000;

Keyset Pagination (USE THIS)

sql

-- FAST: Seeks directly to the right position
SELECT * FROM posts
WHERE created_at < '2024-01-15 10:30:00'
ORDER BY created_at DESC
LIMIT 10;

-- Client passes the last seen value for next page

Partial Indexes

sql

-- Only index the subset you actually query
CREATE INDEX idx_active_orders ON orders(customer_id)
WHERE status = 'active';

-- Smaller index, faster maintenance
-- Only useful for queries that include WHERE status = 'active'

References

•"SQL Performance Explained" - Markus Winand
•use-the-index-luke.com - Free online resource
•modern-sql.com - Modern SQL features