[Proposal] Support Limit Clause Push Down For ODBC Table And MySQL Table

[HappenLee]

Motivation

At present, Doris do not push down limit operation to Exteranl Table Engine.It may serious performance problems in some scenarios. For example, this is a Exteranl Table of Mysql:

CREATE EXTERNAL TABLE `baseall_mysql` (
  `k1` tinyint(4) NULL COMMENT "",
  `k2` smallint(6) NULL COMMENT "",
  `k3` int(11) NULL COMMENT "",
  `k4` bigint(20) NULL COMMENT "",
  `k5` decimal(9, 3) NULL COMMENT "",
  `k6` char(5) NULL COMMENT "",
  `k10` date NULL COMMENT "",
  `k11` datetime NULL COMMENT "",
  `k7` varchar(20) NULL COMMENT "",
  `k8` double NULL COMMENT "",
  `k9` float NULL COMMENT ""
) ENGINE=MYSQL
COMMENT "MYSQL"
PROPERTIES (
"host" = "192.168.1.1",
"port" = "8086",
"user" = "root",
"password" = "123456",
"database" = "test",
"table" = "base"
);

This table have 60000 row in external table.

We do query like:

select * from baseall_mysql limit 10;

Now Doris do not push down limit clause of mysql table, so Doris fetch all row in baseall_mysql and get top 10 of the table.it is cost unnecessary network and mem in MySQL Scan Node / ODBC Scan Node.

In my test:
this query cost 2.1s but push down the limit clause, it only take 0.21s.

So we need to support limit clause push down for ODBC Table and MySQL Table.