Add support for substr for static strings.

[kuba--]

Signed-off-by: kuba-- kuba@sourced.tech

So far vitess supports substring for columns_names. This PR adds support for static string values, so you can:

SELECT SUBSTRING("foo", 1, 2)

[sougou]

Change is good. One minor nit.

[sougou]

Nit: column->column/val

[kuba--]

I thought about it, but if I reuse ColName , e.g.:

SUBSTR openb STRING ',' value_expression closeb
  {
    $$ = &SubstrExpr{Name: &ColName{Name: NewColIdent($3)}, From: $5, To: nil}
  }

How could I figure out later if it's a static string or column name? How could I stop walkSubtree in this case?
Maybe instead of having extra StrVal *SQLVal, I'll add some boolean member (static/resolved/...) to mark it's a static string expression.

[sougou]

Oh. You misunderstood :). I just wanted the SubstrExpr comment to be updated to say that the input can be a column or a value, to reflect the code change.

[kuba--]

Ok, how about now - take a look.