Add MGET, move commands description to a separate file

Author: biozz

COMMANDS.md

@@ -0,0 +1,20 @@
+# Commands
+
+This is a list of supported Redis commands and how they are implemented.
+
+The statuses are one of these:
+
+- `stable` - it works as expected, can be used in production
+- `limited-stable` - only parts of the Redis implementation are available, but they work as expected and can be used in production
+- `beta` - it has been tested, but use with caution
+- `alpha` - experimental, only for developers
+
+| Command |      Status      | Comment                                                                                                                                             |
+|:-------:|:----------------:|:----------------------------------------------------------------------------------------------------------------------------------------------------|
+|  `DEL`  |     `stable`     | `DELETE FROM ... WHERE key = ...`                                                                                                                   |
+|  `GET`  |     `stable`     | `SELECT value FROM ... WHERE key = ...`                                                                                                             |
+| `KEYS`  | `limited-stable` | although, only `*`-globs are supported with `ILIKE` and `%`, because it is the most common scenario for me (to select keys starting with something) |
+| `MGET`  |     `alpha`      | `SELECT value FROM ... WHERE key in (...)`, it might be slow for thousands of keys                                                                  |
+| `PING`  |     `stable`     | `SELECT 1`                                                                                                                                          |
+| `QUIT`  |     `stable`     | N/A (provided by `redcon`)                                                                                                                          |
+|  `SET`  |     `stable`     | `INSERT` or `UPDATE`, i.e. ["UPSERT" method](https://www.postgresql.org/docs/14/sql-insert.html)                                                    |

README.md

@@ -47,14 +47,9 @@ So I decided to implement PostgreSQL backend for Redis' interface.
 
 When you first start postgredis, it will create a table with two columns: unique `key` and `value`. They both are in [text](https://www.postgresql.org/docs/14/datatype-character.html) format. Unique `key` helps with concise "upsert" queries. Thanks to [pgx](https://github.com/jackc/pgx) I don't have think about how to maintain a connection, I just start a [pool](https://github.com/jackc/pgx/tree/master/pgxpool). There is also a logging of sql queries, which is good for debugging.
 
-These Redis commands are supported:
-
-- `SET` - `INSERT` or `UPDATE`, i.e. ["UPSERT" method](https://www.postgresql.org/docs/14/sql-insert.html)
-- `GET` - `SELECT value FROM ... WHERE key = ...`
-- `DEL` - `DELETE FROM ... WHERE key = ...`
-- `KEYS` - although, only `*`-globs are supported with `ILIKE` and `%`, because it is the most common scenario for me (to select keys starting with something)
-- `PING` - `SELECT 1`
-- `QUIT`
+## Supported commands
+
+See COMMANDS.md.
 
 ## Future plans
 

main.go

@@ -68,7 +68,7 @@ func (s *Server) Handler(conn redcon.Conn, cmd redcon.Command) {
 			),
 		)
 		if err != nil {
-			fmt.Println(err)
+			log.Println(err)
 			conn.WriteError(err.Error())
 			return
 		}
@@ -81,11 +81,49 @@ func (s *Server) Handler(conn redcon.Conn, cmd redcon.Command) {
 		var val string
 		err := s.pool.QueryRow(context.Background(), fmt.Sprintf("SELECT value FROM %s WHERE key = '%s'", *table, cmd.Args[1])).Scan(&val)
 		if err != nil {
-			fmt.Println(err)
+			log.Println(err)
 			conn.WriteNull()
 		} else {
 			conn.WriteBulk([]byte(val))
 		}
+	case "mget":
+		if len(cmd.Args) < 2 {
+			conn.WriteError("ERR wrong number of arguments for '" + string(cmd.Args[0]) + "' command")
+			return
+		}
+		quotedKeys := make([]string, len(cmd.Args[1:]))
+		for i, arg := range cmd.Args[1:] {
+			quotedKeys[i] = fmt.Sprintf("'%s'", arg)
+		}
+		query := fmt.Sprintf("SELECT key, value FROM %s WHERE key in (%s)", *table, strings.Join(quotedKeys, ","))
+		log.Println(query)
+		rows, err := s.pool.Query(context.Background(), query)
+		if err != nil {
+			conn.WriteError(err.Error())
+			return
+		}
+		conn.WriteArray(len(cmd.Args[1:]))
+		keysFromDb := make(map[string]string)
+		for rows.Next() {
+			var (
+				key   string
+				value string
+			)
+			err = rows.Scan(&key, &value)
+			if err != nil {
+				conn.WriteError(err.Error())
+				return
+			}
+			keysFromDb[key] = value
+		}
+		for _, key := range cmd.Args[1:] {
+			value, ok := keysFromDb[string(key)]
+			if !ok {
+				conn.WriteNull()
+				continue
+			}
+			conn.WriteAny(value)
+		}
 	case "del":
 		if len(cmd.Args) != 2 {
 			conn.WriteError("ERR wrong number of arguments for '" + string(cmd.Args[0]) + "' command")