Skip sending metadata when possible for binary protocol

This task goal is to permit not sending metadata every time

What MySQL has done in version 8

MySQL has changed the protocol to permit skipping metadata for version 8.
see https://dev.mysql.com/doc/refman/8.0/en/c-api-optional-metadata.html
and https://dev.mysql.com/worklog/task/?id=8134

In description MySQL task indicate some big improvement:

• METADATA_FULL : 3.48w TPS, Net send 113M
• METADATA_IGNORE: 13.8w TPS, Net send 30M

MySQL implementation send metadata (or not) according to resultset_metadata variable.
Associate capability is CLIENT_OPTIONAL_RESULTSET_METADATA (1UL << 25)

When resultset_metadata = 0 (NONE) no metadata is send for COM_STMT_PREPARE/COM_STMT_EXECUTE/COM_QUERY

Exchange format change

Column count packet

There is a "new metadata_follows flag"

* int<lenenc> column count

* int<1> metadata_follows flag (0x00: No metadata, 0x01: metadata follows)

When metadata_follows = 0x00, protocol skip column informations packet (continuing by an EOF_packet if CLIENT_DEPRECATE_EOF capability is not set or rows if CLIENT_DEPRECATE_EOF is set).
be careful not to rely on mysql documentation that indicate them in a wrong order

COM_PREPARE response

COM_STMT_PREPARE_OK:

* int<1> 0x00 COM_STMT_PREPARE_OK header

* int<4> statement id

* int<2> number of columns in the returned result set (or 0 if statement does not return result set)

* int<2> number of prepared statement parameters ('?' placeholders)

* string<1> -not used-

* int<2> number of warnings

* if capabilities & CLIENT_OPTIONAL_RESULTSET_METADATA

**  int<1> metadata_follows flag (0x00: No metadata, 0x01: metadata follows)

}

if metadata is set to 0x00, column definition packet that decribe each parameter and each column are skipped.

Requirement:

Connectors need metadata to parse resultset.
Proposal is to offer this functionality, but in a more modular way.

What do connectors need:

For first execute of a prepared statement:

• COM_STMT_PREPARE asked WITHOUT metadata.
• COM_STMT_EXECUTE asked WITH METADATA. metadata is cached

For next execute:

• COM_STMT_EXECUTE WITHOUT metadata. use cached metadata

There are two issues with not sending metadata each time:

• Metadata can change over time. Server already automatically reprepares internally, so it knows to resend the metadata even if the client asked not to send the metadata every time.
• commands like "SELECT ?" will have metadata type changing according to parameter. Metadata must then be send each time.

COM_STMT_PREPARE still need to have the possibility to ask for metadata (at least java need that possibility to have metadata (example) without any COM_STMT_EXECUTE execution)

For production environment where DDL never change an option can be set on client level to use that same functionality for COM_QUERY.
Client can then cache metadata and ask for COM_QUERY without metadata.

MySQL doesn't permits modularity, so client request have to add this information.
That can be either by:

• create new commands COM_STMT_PREPARE_WITHOUT_META, COM_STMT_EXECUTE_WITHOUT_META and COM_QUERY_WITHOUT_META
• changing existing COM_STMT_PREPARE, COM_STMT_EXECUTE and COM_QUERY format.