Query Composite Indexes

title: Create Composite Indexes for Multi-Column Queries impact: HIGH impactDescription: 5-10x faster multi-column queries tags: indexes, composite-index, multi-column, query-optimization

Create Composite Indexes for Multi-Column Queries

When queries filter on multiple columns, a composite index is more efficient than separate single-column indexes.

Incorrect (separate indexes require bitmap scan):

-- Two separate indexes
create index orders_status_idx on orders (status);
create index orders_created_idx on orders (created_at);

-- Query must combine both indexes (slower)
select * from orders where status = 'pending' and created_at > '2024-01-01';

Correct (composite index):

-- Single composite index (leftmost column first for equality checks)
create index orders_status_created_idx on orders (status, created_at);

-- Query uses one efficient index scan
select * from orders where status = 'pending' and created_at > '2024-01-01';

Column order matters - place equality columns first, range columns last:

-- Good: status (=) before created_at (>)
create index idx on orders (status, created_at);

-- Works for: WHERE status = 'pending'
-- Works for: WHERE status = 'pending' AND created_at > '2024-01-01'
-- Does NOT work for: WHERE created_at > '2024-01-01' (leftmost prefix rule)

Reference: Multicolumn Indexes