How I Segmented Millions of Users in Just a Few Milliseconds

User segmentation requirement Imagine you need to send a push notification to users who satisfy all of the following conditions: Push notification is enabled User is a VIP Active within the last 30 days Following the Voucher Hot category The traditional approach is to query multiple tables: SELECT DISTINCT u . id FROM users u JOIN user_configs c ON c . user_id = u . id JOIN devices d ON d . user_id = u . id JOIN follows f ON f . user_id = u . id WHERE c . push_optin = 1 AND c . mute = 0 AND d . fcm_token IS NOT NULL AND f . category = 'voucher_hot' AND u . last_active >= NOW () - INTERVAL 30 DAY ; As your user base grows into the millions, every campaign requires joining multiple large tables, filtering millions of records, and repeatedly computing the same audience. Query latency increases significantly, making real-time segmentation increasingly difficult. A Different Approach Instead of querying the database every time, we precompute each boolean attribute as a bitmap. Think of a bitmap as a huge array containing only 0 and 1, where the index corresponds to the user ID. For example, a bitmap representing whether a user has enabled push notifications: User ID : 0 1 2 3 4 5 6 7 Bitmap : 1 0 1 1 0 0 1 1 To check whether user 123 has enabled notifications, simply read bit 123. 1 → enabled 0 → disabled Each bitmap represents exactly one boolean property: bitmap:push_optin bitmap:vip bitmap:active30 bitmap:follow:voucher_hot Memory Usage Bitmap is extremely memory efficient. Each user requires only one bit. For 1 million users: 1.000.000 bits ~ 125.000 bytes ~ 122 KB That means every segment only consumes about 122 KB of Redis memory. Even 100 different segments require only around 12 MB . Finding Intersections Suppose you want all users that are: VIP AND Push Opt-in AND Active30 AND Following Voucher Hot Redis can calculate the result with a single command: BITOP AND result vip push_optin active30 follow_voucher_hot Need the number of matched users? BITCOUNT result No