JAMES-2586 ADR for Posgres mailbox tables structure (linagora#1857)

src/adr/0070-postgresql-adoption.md

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-# 68. Native PostgreSQL adoption
+# 70. Native PostgreSQL adoption
 
 Date: 2023-10-31
 

src/adr/0071-postgresql-mailbox-tables-structure.md

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+# 71.  Postgresql Mailbox tables structure
+
+Date: 2023-12-14
+
+## Status
+
+Implemented
+
+## Context
+
+As described in [ADR-70](link), we are willing to provide a Postgres implementation for Apache James.
+The current document is willing to detail the inner working of the mailbox of the target implementation.
+
+## Decision
+
+![diagram for mailbox tables](img/adr-71-mailbox-tables-diagram.png)
+
+Table list:
+- mailbox
+- mailbox_annotations
+- message
+- message_mailbox
+- subscription
+
+Indexes in table message_mailbox: 
+- message_mailbox_message_id_index (message_id)
+- mailbox_id_mail_uid_index (mailbox_id, message_uid)
+- mailbox_id_is_seen_mail_uid_index (mailbox_id, is_seen, message_uid)
+- mailbox_id_is_recent_mail_uid_index (mailbox_id, is_recent, message_uid)
+- mailbox_id_is_delete_mail_uid_index (mailbox_id, is_deleted, message_uid)
+
+Indexes are used to find records faster.
+
+The table structure is mostly normalized which mitigates storage costs and achieves consistency easily.
+
+Foreign key constraints (mailbox_id in mailbox_annotations, message_id in message_mailbox) help to ensure data consistency. For example, message_id 1 in table message_mailbox could not exist if message_id 1 in table message does not exist  
+
+For some fields, hstore data type are used. Hstore is key-value hashmap data structure. Hstore allows us to model complex data types without the need for complex joins.
+
+Special postgres clauses such as RETURNING, ON CONFLICT are used to ensure consistency without the need of combining multiple queries in a single transaction.
+
+## Consequences
+
+Pros:
+- Indexes could increase query performance significantly
+
+Cons:
+- Too many indexes in a table could reduce the performance of updating data in the table
+
+## Alternatives
+
+## References
+
+- [JIRA](https://issues.apache.org/jira/browse/JAMES-2586)
+- [PostgreSQL](https://www.postgresql.org/)
+
+
+