comparing-database-schemas

What this skill does

# Database Diff Tool

## Overview

Compare database schemas between two environments (development vs. staging, staging vs.

## Prerequisites

- Connection credentials to both source and target databases
- `psql` or `mysql` CLI configured to connect to both environments
- Read access to `information_schema` and `pg_catalog` (PostgreSQL) or `information_schema` (MySQL)
- Permission to run `pg_dump --schema-only` for full schema extraction
- Understanding of which environment is the "source of truth" (typically the migration-managed environment)

## Instructions

1. Extract the full schema from both databases for comparison:
   - PostgreSQL: `pg_dump --schema-only --no-owner --no-privileges -f schema_source.sql source_db` and repeat for target_db
   - MySQL: `mysqldump --no-data --routines --triggers source_db > schema_source.sql`
   - Alternatively, query `information_schema` directly for programmatic comparison

2. Compare tables present in each database:
   - `SELECT table_name FROM information_schema.tables WHERE table_schema = 'public' AND table_catalog = 'source_db' EXCEPT SELECT table_name FROM information_schema.tables WHERE table_schema = 'public' AND table_catalog = 'target_db'`
   - This reveals tables that exist in source but not in target (and vice versa)

3. Compare columns for each shared table:
   - Query `information_schema.columns` from both databases for: `column_name`, `data_type`, `character_maximum_length`, `is_nullable`, `column_default`, `ordinal_position`
   - Flag differences in data type, nullability, default values, and column ordering
   - Detect added columns (in source, not target) and dropped columns (in target, not source)

4. Compare indexes:
   - PostgreSQL: Query `pg_indexes` for `indexname`, `indexdef` on each database
   - MySQL: Query `information_schema.STATISTICS` for `INDEX_NAME`, `COLUMN_NAME`, `NON_UNIQUE`
   - Flag missing, extra, or differently-defined indexes

5. Compare constraints (primary keys, foreign keys, unique, check):
   - Query `information_schema.table_constraints` and `information_schema.key_column_usage`
   - Detect missing foreign keys, changed constraint names, and altered check constraint expressions

6. Compare functions, stored procedures, and triggers:
   - PostgreSQL: Query `pg_proc` for function signatures and `pg_trigger` for trigger definitions
   - MySQL: Query `information_schema.ROUTINES` and `information_schema.TRIGGERS`
   - Compare function bodies for logical differences

7. Compare enum types and custom types (PostgreSQL):
   - Query `pg_type` and `pg_enum` for enum label differences
   - Detect added or removed enum values (note: PostgreSQL only supports adding enum values, not removing)

8. Generate a structured diff report categorizing differences as:
   - **Added**: Objects in source not present in target (require CREATE statements)
   - **Removed**: Objects in target not present in source (require DROP statements, confirm intentional)
   - **Modified**: Objects differing between source and target (require ALTER statements)

9. Generate migration SQL to synchronize the target database to match the source:
   - CREATE TABLE for new tables, ALTER TABLE ADD COLUMN for new columns
   - ALTER TABLE ALTER COLUMN for type changes, ALTER TABLE DROP COLUMN for removed columns
   - CREATE INDEX / DROP INDEX for index differences
   - Include transaction wrapping and rollback-safe operations

10. Validate the generated migration by applying it to a copy of the target database and re-running the diff. The second diff should report zero differences, confirming the migration produces the expected state.

## Output

- **Schema diff report** listing all differences categorized by type (added, removed, modified)
- **Migration SQL script** to synchronize target schema to match source
- **Rollback SQL script** to reverse the migration if needed
- **Side-by-side comparison** of differing object definitions
- **Drift detection summary** highlighting changes not tracked in migration files

## Error Handling

| Error | Cause | Solution |
|-------|-------|---------|
| Connection refused to one database | Network or credential issue on source or target | Verify connection strings; check firewall rules; confirm credentials work with direct `psql` or `mysql` connection |
| Permission denied on `pg_catalog` queries | User lacks read access to system catalogs | Grant `pg_read_all_settings` role; or use `pg_dump --schema-only` which requires fewer privileges |
| False positive differences from default value formatting | PostgreSQL normalizes default expressions differently in different versions | Normalize default value strings before comparison; ignore whitespace differences; compare semantic equivalence |
| Enum type modification blocked | PostgreSQL does not support removing enum values or reordering | Create a new enum type, migrate the column, drop the old type; document this as a multi-step migration |
| Generated migration fails on target | Target has data that violates new constraints | Add data validation queries before constraint creation; backfill default values; handle edge cases in migration |

## Examples

**Detecting schema drift between staging and production**: After 3 months without auditing, the diff reveals: 2 columns added to production manually (not in migrations), 1 index missing from staging, and 3 functions with different implementations. A migration script is generated to bring staging in sync, and the manual production changes are backported into migration files.

**Pre-deployment schema validation**: Before deploying a release with 5 migration files, run the diff between the post-migration staging schema and the expected schema. The diff catches a migration that accidentally dropped a constraint that a later migration depends on. The migration ordering is fixed before production deployment.

**Comparing PostgreSQL schemas across major version upgrade**: Schema extracted from PostgreSQL 14 and compared against PostgreSQL 16 after migration. Diff reveals function signature changes for built-in function calls, updated default values for new parameters, and deprecated syntax in stored procedures. Migration script updates function definitions for the new version.

## Resources

- PostgreSQL information_schema: https://www.postgresql.org/docs/current/information-schema.html
- MySQL information_schema: https://dev.mysql.com/doc/refman/8.0/en/information-schema.html
- pg_dump schema-only mode: https://www.postgresql.org/docs/current/app-pgdump.html
- migra (PostgreSQL schema diff tool): https://github.com/djrobstep/migra
- Skeema (MySQL schema management): https://www.skeema.io/