postgis: create one composite PRIMARY KEY instead of two indices

Create a composite PRIMARY KEY for OSM ID and the unique `id` colum
(serial) instead of a separeate index for `id` and OSM ID. This can save
a few GB for large tables.

OSM ID is first, so lookups for diff imports are still fast. The `id`
makes the index unique as OSM IDs are not unique (e.g. multiple
mappings).

database/postgis/postgis.go

@@ -196,6 +196,13 @@ func (pg *PostGIS) Finish() error {
 }
 
 func createIndex(pg *PostGIS, tableName string, columns []ColumnSpec) error {
+	foundIDCol := false
+	for _, cs := range columns {
+		if cs.Name == "id" {
+			foundIDCol = true
+		}
+	}
+
 	for _, col := range columns {
 		if col.Type.Name() == "GEOMETRY" {
 			sql := fmt.Sprintf(`CREATE INDEX "%s_geom" ON "%s"."%s" USING GIST ("%s")`,
@@ -207,7 +214,11 @@ func createIndex(pg *PostGIS, tableName string, columns []ColumnSpec) error {
 				return err
 			}
 		}
-		if col.FieldType.Name == "id" {
+		if col.FieldType.Name == "id" && foundIDCol {
+			// Create index for OSM ID required for diff updates, but only if
+			// the table does have an `id` column.
+			// The explicit `id` column prevented the creation of our composite
+			// PRIMARY KEY index of id (serial) and OSM ID.
 			sql := fmt.Sprintf(`CREATE INDEX "%s_%s_idx" ON "%s"."%s" USING BTREE ("%s")`,
 				tableName, col.Name, pg.Config.ImportSchema, tableName, col.Name)
 			step := log.Step(fmt.Sprintf("Creating OSM id index on %s", tableName))

database/postgis/spec.go

@@ -43,17 +43,21 @@ func (col *ColumnSpec) AsSQL() string {
 
 func (spec *TableSpec) CreateTableSQL() string {
 	foundIDCol := false
+	pkCols := []string{}
 	for _, cs := range spec.Columns {
 		if cs.Name == "id" {
 			foundIDCol = true
 		}
+		if cs.FieldType.Name == "id" {
+			pkCols = append(pkCols, cs.Name)
+		}
 	}
 
 	cols := []string{}
 	if !foundIDCol {
-		// only add id column if there is no id configured
-		// TODO allow to disable id column?
-		cols = append(cols, "id SERIAL PRIMARY KEY")
+		// Create explicit id column only if there is no id configured.
+		cols = append(cols, "id SERIAL")
+		pkCols = append(pkCols, "id")
 	}
 
 	for _, col := range spec.Columns {
@@ -62,6 +66,12 @@ func (spec *TableSpec) CreateTableSQL() string {
 		}
 		cols = append(cols, col.AsSQL())
 	}
+
+	// Make composite PRIMARY KEY of serial `id` and OSM ID. But only if the
+	// user did not provide a custom `id` colum which might not be unique.
+	if pkCols != nil && !foundIDCol {
+		cols = append(cols, `PRIMARY KEY ("`+strings.Join(pkCols, `", "`)+`")`)
+	}
 	columnSQL := strings.Join(cols, ",\n")
 	return fmt.Sprintf(`
         CREATE TABLE IF NOT EXISTS "%s"."%s" (