What are lookup queries for SQL storage backends

[kevinvalk]

The documentation talks about the lookup store and to me it reads as an ephemeral key value store and for some performance optimizations.

Then in PostgreSQL, MySQL / MariaDB and SQLite sections we have a section called "Lookup queries" in which the following is written:

When PostgreSQL is used as a directory or lookup store, SQL queries can be mapped to lookup ids.

I was unable to grasp how the lookup queries are related to the lookup store in the documentation. Moreover, if I read the response by @mdecimus on #236 (comment) it looks like the Lookup queries section really is only about usage as (external) directory for user management.

Can any one clarify if the Lookup queries are relevant when using Stalwart using in an Internal Directory mode?

[kevinvalk]

Given the answer on #308 (comment) I think the docs could be clarified to avoid using the "lookup store" term as it seems only applicable to external directory usage (maybe the term should be "Usage for external directory"?).

Anyways, hoping for some more clarifications.

[mdecimus]

it seems only applicable to external directory usage (maybe the term should be "Usage for external directory"?).

Directories and lookup stores are unrelated, directories are for authentication while lookup stores are used for other purposes such as rate limiting, storing spam filter weights and more.

When PostgreSQL is used as a directory or lookup store, SQL queries can be mapped to lookup ids.

What this means is that you can query an external directory from within an expression or Sieve script using functions such as key_get which are reserved to access lookup stores.
If you don't use external directories or expressions or Sieve scripts you can ignore the concept of lookup queries.