spanner/spansql: support for Update in parseDMLStmt

[lazeratops]

Is your feature request related to a problem? Please describe.

Currently spannertest seems to be unusable with UPDATE SQL queries. parseDMLStmt() in spansql/parser.go seems to only support DELETE. There is a TODO: Insert, Update comment, but I was unable to find an associated feature request issue for this and not sure if it is planned, hence this request to put it on the roadmap. Relevant line: https://github.com/googleapis/google-cloud-go/blob/master/spanner/spansql/parser.go#L1284

Describe the solution you'd like

I would like for UPDATE DML to be parsed via spansql , which would enable us to properly test these statements via spannertest. Ideally INSERT would be parsed as well, but as UPDATE is more relevant for our immediate purpose I am focusing on that one here.

Describe alternatives you've considered

An alternative would be to update via an Update mutation instead. We will likely go with this solution for the time being, but being forced to change the implementation to a mutation solely to account for missing test functionality does not seem ideal.

Thanks!

[dsymonds]

Syntax types and parsing for UPDATE DML is now done in the spansql package. I'll be working on adding its implementation in spannertest next.

[dsymonds]

Implementation is out for review.

You could test it locally real quick by running commands like these in your repo:

go mod edit -replace cloud.google.com/go/spanner=github.com/dsymonds/google-cloud-go/spanner@spannertest-update-dml
go mod download
go mod tidy
go test ./...

and chime in on #3201 if there's something significantly wrong for your use case.