The logic here was wrong and we were always starting from the start of
slice if an existing buffer was passed in to append to. Instead we need
to actually append from the end of the slice.

Signed-off-by: Dirkjan Bussink <d.bussink@gmail.com>

As described in vitessio#13493 we have various bugs where we transcode inputs
incorrectly and where we lack collation information in the results
returned.

The changes here fix a number of things:

- Ensure that we always return collation information for querypb.Field
  instances. This also updates a lot of the static mocked inputs to
  ensure that they match what real queries would return. Many of the
  changes associated with this part of the fixes is ensuring the mocks are
  expectations are updated properly.
- Introduce an IntroduceExpr in the evalengine. One of the issues
  identified is that we didn't correctly handle hex escaped values and
  we would never see them being unescaped if a charset introducer was
  used. The cause of this is that we directly would change the collation
  information on the expression. This is wrong in the case of a HexNum or
  HexVal, since we loose information we need to transcode those. We
  could work around this for the AST evaluator, but the compiler couldn't
  be fixed as the real underlying value was not available with the actual
  hex encoded value. By introducing a new expression, we can also now
  properly compile this.
- No flags information was sent along with evaluated results. This is
  similar to the collation information, although usually not as
  important for clients. We opted here to fix this nonetheless and match
  MySQL as closely as possible.
- Introduce a `collations.SystemCollation` to use consistently for all
  internal table / column names information. It was using mostly the
  constant for utf8mb3 or no information at all which would be problematic
  as it means we woulnd't know what encoding it was.
- Convert outputs to the connection collation / charset. This was
  entirely missing from Vitess but is behavior that MySQL has. If you
  read from say a `latin1` encoded column with a connection charset of
  `utf8mb4`, the value is converted before it's sent over the wire to
  `utf8mb4`. This fix also applies for the `evalengine` tests where we
  can now test also non UTF-8 compatible encodings, fixing a bunch of
  todos. This uncovered some other small bugs in the `evalengine` where
  we were converting things wrong.
- The rowstreamer would return collation information from the result
  set, but that is now wrong because of the above conversion fixes. This
  means we need to use the actual fields information which we already have
  available to see the proper collation. We can copy all the attributes
  for the PK column.
- To be explicit, it includes a rename from `collations.CollationUtf8ID`
  to `collations.CollationUtf8mb3ID` to be explicit about this collation
  name and match how MySQL is also changing this to be explicit.

Signed-off-by: Dirkjan Bussink <d.bussink@gmail.com>

This removes the runtime regexp compilation as that's not necessary. It
also normalizes the casing to match MySQL to upcase things and adds a
leading `0` if needed.

We move the helpers for this to a new utility package.

Signed-off-by: Dirkjan Bussink <d.bussink@gmail.com>

Signed-off-by: Dirkjan Bussink <d.bussink@gmail.com>

This way we can ensure we always initialize these with the proper
explicit typing information.

Signed-off-by: Dirkjan Bussink <d.bussink@gmail.com>

We know this value always based on whether the type is -1 or a valid
value, so we don't need the additional boolean value here.

Signed-off-by: Dirkjan Bussink <d.bussink@gmail.com>

With the type information we can better decide if we need to print the
collation change or not, since it's only needed for textual columns.

This also fixes a number of cases where we used to use the weight_string
but we no longer need to since we now pass through the correct type
information in more cases.

Signed-off-by: Dirkjan Bussink <d.bussink@gmail.com>

Signed-off-by: Dirkjan Bussink <d.bussink@gmail.com>

Signed-off-by: Dirkjan Bussink <d.bussink@gmail.com>

Signed-off-by: Dirkjan Bussink <d.bussink@gmail.com>

Signed-off-by: Dirkjan Bussink <d.bussink@gmail.com>

Signed-off-by: Dirkjan Bussink <d.bussink@gmail.com>

…ncoding-issues

Signed-off-by: Andres Taylor <andres@planetscale.com>

…n the query send down

Signed-off-by: Harshit Gangal <harshit@planetscale.com>