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# Database guides
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## Database Reviews
- If you're creating a database MR for review, check out our [Database review guidelines ](../database_review.md ).
It provides an introduction on database-related changes, migrations, and complex SQL queries.
- If you're a database reviewer or want to become one, check out our [introduction to reviewing database changes ](database_reviewer_guidelines.md ).
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## Tooling
- [Understanding EXPLAIN plans ](../understanding_explain_plans.md )
- [explain.depesz.com ](https://explain.depesz.com/ ) or [explain.dalibo.com ](https://explain.dalibo.com/ ) for visualizing the output of `EXPLAIN`
- [pgFormatter ](http://sqlformat.darold.net/ ) a PostgreSQL SQL syntax beautifier
## Migrations
- [What requires downtime? ](../what_requires_downtime.md )
- [SQL guidelines ](../sql.md ) for working with SQL queries
- [Migrations style guide ](../migration_style_guide.md ) for creating safe SQL migrations
- [Testing Rails migrations ](../testing_guide/testing_migrations_guide.md ) guide
- [Post deployment migrations ](../post_deployment_migrations.md )
- [Background migrations ](../background_migrations.md )
- [Swapping tables ](../swapping_tables.md )
- [Deleting migrations ](../deleting_migrations.md )
## Debugging
- Tracing the source of an SQL query using query comments with [Marginalia ](../database_query_comments.md )
- Tracing the source of an SQL query in Rails console using [Verbose Query Logs ](https://guides.rubyonrails.org/debugging_rails_applications.html#verbose-query-logs )
## Best practices
- [Adding database indexes ](../adding_database_indexes.md )
- [Foreign keys & associations ](../foreign_keys.md )
- [Adding a foreign key constraint to an existing column ](add_foreign_key_to_existing_column.md )
- [`NOT NULL` constraints ](not_null_constraints.md )
- [Strings and the Text data type ](strings_and_the_text_data_type.md )
- [Single table inheritance ](../single_table_inheritance.md )
- [Polymorphic associations ](../polymorphic_associations.md )
- [Serializing data ](../serializing_data.md )
- [Hash indexes ](../hash_indexes.md )
- [Storing SHA1 hashes as binary ](../sha1_as_binary.md )
- [Iterating tables in batches ](../iterating_tables_in_batches.md )
- [Insert into tables in batches ](../insert_into_tables_in_batches.md )
- [Ordering table columns ](../ordering_table_columns.md )
- [Verifying database capabilities ](../verifying_database_capabilities.md )
- [Database Debugging and Troubleshooting ](../database_debugging.md )
- [Query Count Limits ](../query_count_limits.md )
- [Creating enums ](../creating_enums.md )
## Case studies
- [Database case study: Filtering by label ](../filtering_by_label.md )
- [Database case study: Namespaces storage statistics ](../namespaces_storage_statistics.md )
