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Migration Patterns

TL;DR

Common migration patterns for safely evolving ClickHouse schemas include expand and contract (for renames and type changes), branch + exchange (for full rewrites), and versioning + canary (for testing in production). These patterns help you safely execute schema changes while maintaining system availability and data consistency.

Expand and Contract

The expand and contract pattern is a safe way to rename columns or change data types. You add a new column with the desired name or type, update ingestion to write to both columns, validate the change, and then drop the old column.

Phase 1: Expand (Add New Column)

ALTER TABLE events ADD COLUMN new_column_name NewDataType DEFAULT defaultValue;

Phase 2: Migrate (Write to Both)

-- Update ingestion to write to both columns
PreviousSchema Change Costs
NextDeveloper Expectations

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Migration Patterns | ClickHouse for Developers