mysql2postgres/MIGRATION_WORKFLOW.md

MySQL to PostgreSQL Migration Workflow

Overview

This tool implements consolidation-based incremental migration for two tables:

• RAWDATACOR: Raw sensor measurements
• ELABDATADISP: Elaborated/calculated data

Both tables use consolidation keys to group and migrate data efficiently.

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Migration Modes

1. Full Migration

Initial migration of all historical data, migrating one partition at a time.

# Migrate all partitions for all tables
python main.py migrate full

# Migrate specific table
python main.py migrate full --table RAWDATACOR

# Migrate specific partition (year-based)
python main.py migrate full --table ELABDATADISP --partition 2024

# Dry-run to see what would be migrated
python main.py migrate full --dry-run

Characteristics:

• Migrates data partition by partition (year-based)
• Uses consolidation groups for efficiency
• Tracks progress in migration_state table (PostgreSQL)
• Can resume from last completed partition if interrupted
• Uses mysql_max_id optimization for performance

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2. Incremental Migration

Sync only new data since the last migration.

# Migrate new data for all tables
python main.py migrate incremental

# Migrate specific table
python main.py migrate incremental --table ELABDATADISP

# Dry-run to check what would be migrated
python main.py migrate incremental --dry-run

Characteristics:

• Uses consolidation keys to identify new records:
  • (UnitName, ToolNameID, EventDate, EventTime)
• Tracks last migrated key in migration_state table
• Optimized with min_mysql_id filter for performance
• Handles duplicates with ON CONFLICT DO NOTHING
• Perfect for scheduled jobs (cron, systemd timers)

How it works:

1. Retrieves last_key from migration_state table
2. Gets MAX(mysql_max_id) from PostgreSQL table for optimization
3. Queries MySQL: WHERE id > max_mysql_id AND (key_tuple) > last_key
4. Migrates new consolidation groups
5. Updates migration_state with new last_key

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Consolidation Keys

Both tables use consolidation to group multiple measurements into a single JSONB record.

Consolidation Key Structure

(UnitName, ToolNameID, EventDate, EventTime)

Why Consolidation?

Instead of migrating individual sensor readings, we:

1. Group all measurements for the same (unit, tool, date, time)
2. Transform 16-25 columns into structured JSONB
3. Migrate as a single consolidated record

Example:

MySQL has 10 rows for (Unit1, Tool1, 2024-01-01, 10:00:00):

id  | UnitName | ToolNameID | EventDate  | EventTime | Val0  | Val1  | ...
1   | Unit1    | Tool1      | 2024-01-01 | 10:00:00  | 23.5  | 45.2  | ...
2   | Unit1    | Tool1      | 2024-01-01 | 10:00:00  | 23.6  | 45.3  | ...
...

PostgreSQL gets 1 consolidated record:

{
  "unit_name": "Unit1",
  "tool_name_id": "Tool1",
  "event_timestamp": "2024-01-01 10:00:00",
  "measurements": {
    "0": {"value": 23.5, "unit": "°C"},
    "1": {"value": 45.2, "unit": "bar"},
    ...
  },
  "mysql_max_id": 10
}

---

State Management

Migration State Table

The migration_state table in PostgreSQL tracks migration progress:

CREATE TABLE migration_state (
    table_name VARCHAR(50),
    partition_name VARCHAR(50),
    last_key JSONB,  -- Last migrated consolidation key
    started_at TIMESTAMP,
    completed_at TIMESTAMP,
    total_rows INTEGER,
    status VARCHAR(20)
);

State Records

• Per-partition state: Tracks each partition's progress

  • Example: ('rawdatacor', '2024', {...}, '2024-01-15 10:30:00', 'completed', 1000000)

• Global state: Tracks overall incremental migration position

  • Example: ('rawdatacor', '_global', {...}, NULL, NULL, 0, 'in_progress')

Checking State

-- View all migration state
SELECT * FROM migration_state ORDER BY table_name, partition_name;

-- View global state (for incremental migration)
SELECT * FROM migration_state WHERE partition_name = '_global';

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Performance Optimization

MySQL ID Filter

The incremental migration uses MAX(mysql_max_id) from PostgreSQL to filter MySQL queries:

SELECT UnitName, ToolNameID, EventDate, EventTime
FROM RAWDATACOR
WHERE id > 267399536  -- max_mysql_id from PostgreSQL
  AND (UnitName, ToolNameID, EventDate, EventTime) > (?, ?, ?, ?)
GROUP BY UnitName, ToolNameID, EventDate, EventTime
ORDER BY UnitName, ToolNameID, EventDate, EventTime
LIMIT 10000

Why this is fast:

• Uses PRIMARY KEY index on id to skip millions of already-migrated rows
• Tuple comparison only applied to filtered subset
• Avoids full table scans

Consolidation Group Batching

Instead of fetching individual rows, we:

1. Fetch 10,000 consolidation keys at a time
2. For each key, fetch all matching rows from MySQL
3. Transform and insert into PostgreSQL
4. Update state every batch

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Recommended Workflow

Initial Setup (One-time)

# 1. Configure .env file
cp .env.example .env
nano .env

# 2. Create PostgreSQL schema
python main.py setup --create-schema

# 3. Run full migration
python main.py migrate full

Daily Incremental Sync

# Run incremental migration (via cron or manual)
python main.py migrate incremental

Cron example (daily at 2 AM):

0 2 * * * cd /path/to/mysql2postgres && python main.py migrate incremental >> /var/log/migration.log 2>&1

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Resuming Interrupted Migrations

Full Migration

If interrupted, full migration resumes from the last completed partition:

# First run: migrates partitions 2014, 2015, 2016... (interrupted after 2020)
python main.py migrate full --table RAWDATACOR

# Resume: continues from partition 2021
python main.py migrate full --table RAWDATACOR

Incremental Migration

Incremental migration uses the last_key from migration_state:

# Always safe to re-run - uses ON CONFLICT DO NOTHING
python main.py migrate incremental

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Syncing Migration State

If migration_state becomes out of sync with actual data, use the sync utility:

# Sync migration_state with actual PostgreSQL data
python scripts/sync_migration_state.py

This finds the most recent row (by created_at) and updates migration_state._global.

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Monitoring

Check Progress

# View migration state
psql -h localhost -U postgres -d migrated_db -c \
  "SELECT table_name, partition_name, status, total_rows, completed_at
   FROM migration_state
   ORDER BY table_name, partition_name"

Verify Row Counts

-- PostgreSQL
SELECT COUNT(*) FROM rawdatacor;
SELECT COUNT(*) FROM elabdatadisp;

-- Compare with MySQL
-- mysql> SELECT COUNT(DISTINCT UnitName, ToolNameID, EventDate, EventTime) FROM RAWDATACOR;

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Common Issues

"No previous migration found"

Cause: Running incremental migration before full migration

Solution: Run full migration first

python main.py migrate full

"Duplicate key value violates unique constraint"

Cause: Data already exists (shouldn't happen with ON CONFLICT DO NOTHING)

Solution: Migration handles this automatically - check logs for details

Slow Incremental Migration

Cause: MAX(mysql_max_id) query is slow (~60 seconds on large tables)

Solution: This is expected and only happens once at start. The MySQL queries are instant thanks to the optimization.

Alternative: Create an index on mysql_max_id in PostgreSQL (uses disk space):

CREATE INDEX idx_rawdatacor_mysql_max_id ON rawdatacor (mysql_max_id DESC);
CREATE INDEX idx_elabdatadisp_mysql_max_id ON elabdatadisp (mysql_max_id DESC);

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Key Technical Details

Tuple Comparison in MySQL

MySQL supports lexicographic tuple comparison:

WHERE (UnitName, ToolNameID, EventDate, EventTime) > ('Unit1', 'Tool1', '2024-01-01', '10:00:00')

This is equivalent to:

WHERE UnitName > 'Unit1'
   OR (UnitName = 'Unit1' AND ToolNameID > 'Tool1')
   OR (UnitName = 'Unit1' AND ToolNameID = 'Tool1' AND EventDate > '2024-01-01')
   OR (UnitName = 'Unit1' AND ToolNameID = 'Tool1' AND EventDate = '2024-01-01' AND EventTime > '10:00:00')

But much more efficient!

Partitioning in PostgreSQL

Tables are partitioned by year (2014-2031):

CREATE TABLE rawdatacor_2024 PARTITION OF rawdatacor
  FOR VALUES FROM (2024) TO (2025);

PostgreSQL automatically routes INSERTs to the correct partition based on event_year.

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Summary

1. Full migration: One-time initial load, partition by partition
2. Incremental migration: Daily sync of new data using consolidation keys
3. State tracking: PostgreSQL migration_state table
4. Performance: mysql_max_id filter + consolidation batching
5. Resumable: Both modes can resume from interruptions
6. Safe: ON CONFLICT DO NOTHING prevents duplicates