add support for sharding in clickhouse (#1)

database/clickhouse/README.md

@@ -7,12 +7,13 @@
 | `x-migrations-table`| Name of the migrations table |
 | `x-migrations-table-engine`| Engine to use for the migrations table, defaults to TinyLog |
 | `x-cluster-name` | Name of cluster for creating `schema_migrations` table cluster wide |
+| `x-sharding` | Use distributed `schema_migrations` table in cluster with sharding enabled |
 | `database` | The name of the database to connect to |
 | `username` | The user to sign in as |
 | `password` | The user's password |
 | `host` | The host to connect to. |
 | `port` | The port to bind to. |
-| `x-multi-statement` | false | Enable multiple statements to be ran in a single migration (See note below) |
+| `x-multi-statement` | Enable multiple statements to be ran in a single migration |
 
 ## Notes
 
@@ -23,3 +24,4 @@
 * Clickhouse cluster mode is not officially supported, since it's not tested right now, but you can try enabling `schema_migrations` table replication by specifying a `x-cluster-name`:
   * When `x-cluster-name` is specified, `x-migrations-table-engine` also should be specified. See the docs regarding [replicated table engines](https://clickhouse.tech/docs/en/engines/table-engines/mergetree-family/replication/#table_engines-replication).
   * When `x-cluster-name` is specified, only the `schema_migrations` table is replicated across the cluster. You still need to write your migrations so that the application tables are replicated within the cluster.
+  * When `x-cluster-name` is specified, you can use `x-sharding` if your cluster supports multiple shards. The `schema_migrations` table will be created as distributed table so that is is accessible on any shard.

database/clickhouse/clickhouse.go

@@ -34,6 +34,7 @@ type Config struct {
 	MigrationsTableEngine string
 	MultiStatementEnabled bool
 	MultiStatementMaxSize int
+	Sharding              bool
 }
 
 func init() {
@@ -101,6 +102,7 @@ func (ch *ClickHouse) Open(dsn string) (database.Driver, error) {
 			ClusterName:           purl.Query().Get("x-cluster-name"),
 			MultiStatementEnabled: purl.Query().Get("x-multi-statement") == "true",
 			MultiStatementMaxSize: multiStatementMaxSize,
+			Sharding:              purl.Query().Get("x-sharding") == "true",
 		},
 	}
 
@@ -232,21 +234,32 @@ func (ch *ClickHouse) ensureVersionTable() (err error) {
 	}
 
 	// if not, create the empty migration table
-	if len(ch.config.ClusterName) > 0 {
-		query = fmt.Sprintf(`
-			CREATE TABLE %s ON CLUSTER %s (
-				version    Int64,
-				dirty      UInt8,
-				sequence   UInt64
-			) Engine=%s`, ch.config.MigrationsTable, ch.config.ClusterName, ch.config.MigrationsTableEngine)
+	return ch.createVersionTable()
+}
+
+func (ch *ClickHouse) createVersionTable() (err error) {
+	if len(ch.config.ClusterName) == 0 {
+		return ch.createBasicVersionTable(ch.config.MigrationsTable)
+	}
+
+	if ch.config.Sharding {
+		localTableName := ch.config.MigrationsTable + "_local"
+		if err := ch.createClusterWideVersionTable(localTableName); err != nil {
+			return err
+		}
+		return ch.createDistributedVersionTable(ch.config.MigrationsTable, localTableName)
 	} else {
-		query = fmt.Sprintf(`
-			CREATE TABLE %s (
-				version    Int64,
-				dirty      UInt8,
-				sequence   UInt64
-			) Engine=%s`, ch.config.MigrationsTable, ch.config.MigrationsTableEngine)
+		return ch.createClusterWideVersionTable(ch.config.MigrationsTable)
 	}
+}
+
+func (ch *ClickHouse) createBasicVersionTable(tableName string) (err error) {
+	query := fmt.Sprintf(`
+		CREATE TABLE %s (
+			version    Int64,
+			dirty      UInt8,
+			sequence   UInt64
+		) Engine=%s`, tableName, ch.config.MigrationsTableEngine)
 
 	if strings.HasSuffix(ch.config.MigrationsTableEngine, "Tree") {
 		query = fmt.Sprintf(`%s ORDER BY sequence`, query)
@@ -258,6 +271,36 @@ func (ch *ClickHouse) ensureVersionTable() (err error) {
 	return nil
 }
 
+func (ch *ClickHouse) createClusterWideVersionTable(tableName string) (err error) {
+	query := fmt.Sprintf(`
+		CREATE TABLE %s ON CLUSTER %s (
+			version    Int64,
+			dirty      UInt8,
+			sequence   UInt64
+		) Engine=%s`, tableName, ch.config.ClusterName, ch.config.MigrationsTableEngine)
+	if strings.HasSuffix(ch.config.MigrationsTableEngine, "Tree") {
+		query = fmt.Sprintf(`%s ORDER BY sequence`, query)
+	}
+
+	if _, err := ch.conn.Exec(query); err != nil {
+		return &database.Error{OrigErr: err, Query: []byte(query)}
+	}
+	return nil
+}
+
+func (ch *ClickHouse) createDistributedVersionTable(distTableName, localTableName string) (err error) {
+	query := fmt.Sprintf(`
+		CREATE TABLE %s ON CLUSTER %s
+		AS %s
+		ENGINE = Distributed(%s, %s, %s, cityHash64(sequence))`,
+		distTableName, ch.config.ClusterName, localTableName, ch.config.ClusterName, ch.config.DatabaseName, localTableName)
+
+	if _, err := ch.conn.Exec(query); err != nil {
+		return &database.Error{OrigErr: err, Query: []byte(query)}
+	}
+	return nil
+}
+
 func (ch *ClickHouse) Drop() (err error) {
 	query := "SHOW TABLES FROM " + ch.config.DatabaseName
 	tables, err := ch.conn.Query(query)