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skills/configuring-transaction-isolation/references/database-defaults.md

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+# Database-Specific Defaults
+
+## PostgreSQL
+
+Default: `ReadCommitted`
+
+Supported levels:
+
+- `Serializable` (strictest)
+- `RepeatableRead`
+- `ReadCommitted` (default)
+
+Notes:
+
+- PostgreSQL uses true Serializable isolation (not snapshot isolation)
+- May throw serialization errors under high concurrency
+- Excellent MVCC implementation reduces conflicts
+
+## MySQL
+
+Default: `RepeatableRead`
+
+Supported levels:
+
+- `Serializable`
+- `RepeatableRead` (default)
+- `ReadCommitted`
+- `ReadUncommitted` (not recommended)
+
+Notes:
+
+- InnoDB engine required for transaction support
+- Uses gap locking in RepeatableRead mode
+- Serializable adds locking to SELECT statements
+
+## SQLite
+
+Default: `Serializable`
+
+Supported levels:
+
+- `Serializable` (only level - database-wide lock)
+
+Notes:
+
+- Only one writer at a time
+- No true isolation level configuration
+- Best for single-user or low-concurrency applications
+
+## MongoDB
+
+Default: `Snapshot` (similar to RepeatableRead)
+
+Supported levels:
+
+- `Snapshot` (equivalent to RepeatableRead)
+- `Majority` read concern
+
+Notes:
+
+- Different isolation model than SQL databases
+- Uses write-ahead log for consistency
+- Replica set required for transactions