lazy data serialization

[tanner0101]

Currently PostgresData values are serialized to the expected data format upon initialization. For example:

import Foundation
import PostgresNIO

let date = PostgresData(date: Date())

Upon initialization, date contains TIMESTAMPTZ formatted bytes ready for querying. The Date supplied is discarded.

While this approach is simple, it has a draw back. Take the following query:

SELECT $1::JSON as "date"

If we attempted to pass the date data from above to this query, we would get a warning followed by an error:

2019-10-18T15:12:42-0400 warning: bind $1 type (TIMESTAMPTZ) does not match expected parameter type (JSON)
2019-10-18T15:12:42-0400 error: invalid byte sequence for encoding "UTF8": 0x00 (report_invalid_encoding)

It makes sense that TIMESTAMPTZ should not be convertible to JSON, however, the original Date object we had earlier does know how to convert to JSON. It can do so via Codable and the JSONEncoder.

If PostgresData could support lazy serialization of Swift types, then we could support passing a Date to this query dynamically.

[tanner0101]

Although #63 did a lot to help this issue, lazy data serialization could still have some benefits. Postgres is able to convert between most types itself, but there are some that it can't. For example: JSON[] to JSONB. This can be an issue if you store a property like users: [User] on your model and set the table's data type to JSONB or JSON[]. PostgresData must determine when it gets [User] whether to convert it to JSONB or JSON[] immediately. Depending on which decision is made, the resulting data may be incompatible with the column's expected data type.

If data could serialize lazily, then [User] could theoretically know how to serialize to both JSONB and JSON[]. The Swift array would be passed through the query chain until the desired data type has been resolved and the array can be asked to serialize.

The one major caveat here as discovered in #63 is that this will require an extra round trip for each query since the client must wait until the DB returns its desired parameter types before sending the bind command.

[fabianfett]

@gwynne I think this is something we should not support... WDYT?

[fabianfett]

I do think this is a non goal right now. Let's close this for now.