Postgres: Support multiple endpoints

[mxey]

The Postgres storage driver can only connect to a single host. It would be useful for clustering Postgres if it could pick the master among multiple hosts.

That is a limitation of the library lib/pq that is used in it. The library is in maintenance mode and this feature is unlikely to be added. They recommend https://github.com/jackc/pgx as a replacement.

pgx supports target_session_attrs, like libpq does:

target_session_attrs
If this parameter is set to read-write, only a connection in which read-write transactions are accepted by default is considered acceptable. The query SHOW transaction_read_only will be sent upon any successful connection; if it returns on, the connection will be closed. If multiple hosts were specified in the connection string, any remaining servers will be tried just as if the connection attempt had failed. The default value of this parameter, any, regards all connections as acceptable.