Adding godocs to newNonCoveringLookupBuilder and changing it to retur…

…n an error (instead of panic'ing) if a primary index is passed in

go/libraries/doltcore/sqle/index/index_lookup.go

@@ -287,7 +287,7 @@ func NewLookupBuilder(
 		// the current schema. In those cases, the primary index is still the best we have, so go ahead and use it.
 		return newCoveringLookupBuilder(base), nil
 	default:
-		return newNonCoveringLookupBuilder(s, base), nil
+		return newNonCoveringLookupBuilder(s, base)
 	}
 }
 
@@ -306,7 +306,17 @@ func newCoveringLookupBuilder(b *baseLookupBuilder) *coveringLookupBuilder {
 	}
 }
 
-func newNonCoveringLookupBuilder(s *durableIndexState, b *baseLookupBuilder) *nonCoveringLookupBuilder {
+// newNonCoveringLookupBuilder returns a LookupBuilder that uses the specified index state and
+// base lookup builder to create a nonCoveringLookupBuilder that uses the secondary index (from
+// |b|) to find the PK row identifier, and then uses that PK to look up the complete row from
+// the primary index (from |s|). If a baseLookupBuilder built on the primary index is passed in,
+// this function returns an error.
+func newNonCoveringLookupBuilder(s *durableIndexState, b *baseLookupBuilder) (*nonCoveringLookupBuilder, error) {
+	if b.idx.ID() == "PRIMARY" {
+		return nil, fmt.Errorf("incompatible index passed to newNonCoveringLookupBuilder: " +
+			"primary index passed, but only secondary indexes are supported")
+	}
+
 	primary := durable.ProllyMapFromIndex(s.Primary)
 	priKd, _ := primary.Descriptors()
 	tbBld := val.NewTupleBuilder(priKd)
@@ -321,7 +331,7 @@ func newNonCoveringLookupBuilder(s *durableIndexState, b *baseLookupBuilder) *no
 		keyMap:            keyProj,
 		valMap:            valProj,
 		ordMap:            ordProj,
-	}
+	}, nil
 }
 
 var _ LookupBuilder = (*baseLookupBuilder)(nil)
@@ -416,7 +426,7 @@ func (lb *coveringLookupBuilder) NewRowIter(ctx *sql.Context, part sql.Partition
 
 // nonCoveringLookupBuilder constructs row iters for non-covering lookups,
 // where we need to seek on the secondary table for key identity, and then
-// the primary table to fill all requrested projections.
+// the primary table to fill all requested projections.
 type nonCoveringLookupBuilder struct {
 	*baseLookupBuilder