[Postgres] Add support for ltree

[mehcode]

https://www.postgresql.org/docs/current/ltree.html

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• Requires remembering type format preferences and telling postgres about this at prepare/bind time. ltree does not support the binary format.

  https://github.com/launchbadge/sqlx/blob/master/sqlx-core/src/postgres/arguments.rs#L30

[greglearns]

Postgres just committed support for binary format, but it is not released yet: https://commitfest.postgresql.org/24/2242/. In the mean time, you can (easily?) treat ltrees as TEXT which does work over the binary channel using select ltree2text(my_tree) and insert via insert int my_table (my_tree) values ($1::text::ltree).

[mehcode]

You don't need the casts if we implement it correctly. In other words, when I say text format I don't mean the text data type.

Good to see binary support being added. We support 9.6+ at the moment so we could only send in binary if we detect the server version or something.

[jlgerber]

With Postgresql 13.0, it looks like binary support for ltree has landed:

Add support for binary I/O of ltree, lquery, and ltxtquery types (Nino Floris)