sql: only allow altering a column type when an assignment cast is allowed

[andrew-delph]

Fixes: #82416

Release note (sql change): Reject changing column types unless an explicitly cast is provided where cannot automatically cast. This replicates logic in Postgres. Prior to this change, casts such as BOOL to INT were possible without explicitly casting.

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[cockroach-teamcity]

This change is 

[blathers-crl]

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[rafiss]

thanks for your contribution!

Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @andrew-delph and @mgartner)

---

-- commits line 9 at r1:
nit: "postures" -> "postgres"

---

pkg/sql/alter_column_type.go line 342 at r1 (raw file):

		// Validate that the column can be automatically cast to toType without explicit casting.
		if validCast := cast.ValidCast(col.GetType(), toType, cast.ContextAssignment); !validCast {
			return pgerror.WithCandidateCode(

nit: use this constructor:

return errors.WithHintf(  
  pgerror.Newf(  
   pgcode.DatatypeMismatch,  
   "column %q cannot be cast automatically to type %s",  
   col.GetName(),  
   toType.SQLString(),  
  ),  "You might need to specify \"USING %s\".", s,  
)

[andrew-delph]

Looks like there are failing UTs, so I will take a look.

[yuzefovich]

Friendly ping @mgartner

[mgartner]

Thanks for the contribution!

Reviewed 1 of 2 files at r1, 1 of 1 files at r2, 4 of 4 files at r3, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @andrew-delph and @rafiss)

[rafiss]

lgtm!

bors r+

[craig]

Build succeeded!

And happy new year! 🎉

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