fix(postgres): propagate persistent to intermediary queries

[v0idpwn]

In some situations (e.g.: when using enums), sqlx will fire additional queries to fetch data from the database. Currently, these queries use persistent=true, regardless of whether the parent query did. Unfortunately, this means that sqlx can't be used with connection poolers that don't support named prepared statements.

In this patch, I try to propagate the persistent attribute from the parent query to the other queries.

Additionally, this requires a potential re-send of Parse/Describe, as the additional queries will replace the existing one Postgres-side. For example, this is a sequence that could happen (after propagating persistent):

Parse("", "SELECT * FROM mytable")
Describe("")
Sync()
Parse("", "SELECT enumlabel FROM pg_catalog.pg_enum WHERE enumtypid = $1 ORDER BY enumsortorder") Describe("")
Sync()
Bind(...)
Execute(..)
Sync()
Bind(...)
Execute(..)
Sync()

This obviously won't work, as the second Parse message overwrote the unnamed prepared statement. Hence I added a re-preparation of the original query:

Parse("", "SELECT * FROM mytable")
Describe("")
Sync()
Parse("", "SELECT enumlabel FROM pg_catalog.pg_enum WHERE enumtypid = $1 ORDER BY enumsortorder") Describe("")
Sync()
Bind(...)
Execute(..)
Sync()
Parse("", "SELECT * FROM mytable")
Describe("")
Bind(...)
Execute(..)
Sync()

Let me know if I missed any place where this should be done, or if you disagree in the concept.

Is this a breaking change?

I don't believe so.

[abonander]

This kind of situation is likely better handled by disabling prepared statement caching at the connection level, which can be done by setting the statement cache capacity to zero: https://docs.rs/sqlx/latest/sqlx/postgres/struct.PgConnectOptions.html#method.statement_cache_capacity

That might have the same problem of needing to re-prepare the query, though.

It might be better to use the raw_sql interface to fetch this information so we don't have to prepare the user's query twice.

[v0idpwn]

Hi, @abonander, thanks for the review.
I tried setting statement_cache_capacity to zero but sqlx continues using named prepared statements, at least on 0.8.6.

[v0idpwn]

I feel like statement_cache_capacity could set persistent=false at execution time if it's 0, so alongside this PR it would ensure that named prepared statements aren't used ever if statement_cache_capacity == 0