Performing schema changes on production databases comes with significant overhead, such as locking entire tables and potentially causing replication lag in master-slave architectures. For MySQL and its variants, there's a powerful tool that mitigates these issues: pt-online-schema-change.

What is pt-online-schema-change?

pt-online-schema-change is a tool that alters a table's structure without blocking reads or writes, making it ideal for use in production environments.

How It Works

1. Table Copying: It creates an empty copy of the table with the desired alterations.

2. Data Migration: It copies rows from the original table to the new one in small, manageable chunks.

3. Real-time Updates: During the copy process, any modifications to the original table are reflected in the new table using triggers.

4. Atomic Swap: Once copying is complete, it uses an atomic RENAME TABLE operation to switch the old and new tables.

5. Cleanup: Finally, it drops the original table.

Key Features and Safety Measures

• Chunk-based Processing: Data is copied in small chunks, optimized for performance (configurable with --chunk-time).

• Trigger Mechanism: Ensures data consistency during the migration process.

• Foreign Key Handling: Supports methods to update foreign key references after the schema change.

• Safety Checks:

  • Requires a PRIMARY KEY or UNIQUE INDEX in most cases.

  • Detects and respects replication filters.

  • Pauses operations if replicas lag behind.

  • Monitors server load and can pause or abort if thresholds are exceeded.

  • Sets conservative lock wait timeouts to minimize disruption.

  • Careful handling of foreign key constraints.

Considerations

• By default, it doesn't modify the table unless the --execute option is specified.

• It may rename foreign keys and indexes slightly to avoid naming collisions.

• Existing triggers on the table will prevent the tool from working.

Example Usage

#!/bin/bash

USERNAME="root"
PASSWORD="your_password_here"
HOST="localhost"
DATABASE="critical_db"
TABLE="critical_table"
ALTER_STATEMENT="ADD COLUMN guest VARCHAR(45), ADD COLUMN name VARCHAR(100)"

if [[ "$1" == "--execute" ]]; then
    EXECUTE=true
else
    EXECUTE=false
fi

echo "Performing dry run..."
pt-online-schema-change \
    --user="$USERNAME" \
    --password="$PASSWORD" \
    --host="$HOST" \
    D="$DATABASE",t="$TABLE" \
    --alter="$ALTER_STATEMENT" \
    --preserve-triggers \
    --dry-run

if [ "$EXECUTE" = true ]; then
    echo "Applying changes..."
    pt-online-schema-change \
        --user="$USERNAME" \
        --password="$PASSWORD" \
        --host="$HOST" \
        D="$DATABASE",t="$TABLE" \
        --alter="$ALTER_STATEMENT" \
        --preserve-triggers \
        --execute
fi

To use this script:

1. Save it to a file (e.g., alter_table.sh)

2. Make it executable: chmod +x alter_table.sh

3. Run a dry run: ./alter_table.sh

4. Execute the changes: ./alter_table.sh --execute

Conclusion

pt-online-schema-change offers a robust solution for performing schema changes in production environments, minimizing downtime and reducing the risks associated with traditional ALTER TABLE operations.