sql: cannot drop columns after ALTER PRIMARY KEY

[RichardJCai]

This stems from not being able to drop columns that are stored in the PK.
In alter_table.go, we check if the column is stored in the pk, if it is, it disallows dropping the column. For PK we should only be checking if the column is indexed.

 n.tableDesc.PrimaryIndex.ContainsColumnID(colToDrop.ID) {
	return pgerror.Newf(pgcode.InvalidColumnReference,
	"column %q is referenced by the primary key", colToDrop.Name)
}

// ContainsColumnID returns true if the index descriptor contains the specified
// column ID either in its explicit column IDs, the extra column IDs, or the
// stored column IDs.
func (desc *IndexDescriptor) ContainsColumnID(colID ColumnID) bool {
	return desc.RunOverAllColumns(func(id ColumnID) error {
		if id == colID {
			return returnTruePseudoError
		}
		return nil
	}) != nil
}

Example: Cannot drop any columns after running ALTER PRIMARY KEY since the columns become explicitly stored.

[email protected]:51308/movr> create table t(x int);
CREATE TABLE
Time: 3.229ms
[email protected]:51308/movr> alter table t alter primary key using columns (rowid);
NOTICE: primary key changes are finalized asynchronously; further schema changes on this table may be restricted until the job completes
ALTER TABLE
Time: 89.82ms
[email protected]:51308/movr> set sql_safe_updates = false;
SET
Time: 249µs
[email protected]:51308/movr> alter table t drop column x;
ERROR: column "x" is referenced by the primary key
SQLSTATE: 42P10
[email protected]:51308/movr>

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[RichardJCai]

@rohany