请教个问题哈，我们现在 tiflash 是否支持从 tikv 上取部分数据，然后再去 tiflash 上做 join 呢？
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现在应该是不支持
二选一，应该是不支持的。不过一个语句join的两个表，可以一个走tikv，一个走tiflash吧，然后再tidb里join 
tiflash用不了行存数据。所以原因还是存储紧张？可以考虑将tiflash副本放到s3上。
https://docs.pingcap.com/zh/tidb/stable/tiflash-disaggregated-and-s3#tiflash-存算分离架构与-s3-支持
不行
CREATE TABLE users (
id INT PRIMARY KEY,
name VARCHAR(255),
city VARCHAR(255)
);
CREATE TABLE orders (
id INT PRIMARY KEY,
user_id INT,
amount DECIMAL(10, 2),
KEY idx_user_id (user_id)
);
INSERT INTO users (id, name, city) VALUES
(1, 'Alice', 'Beijing'),
(2, 'Bob', 'Shanghai'),
(3, 'Charlie', 'Guangzhou');
INSERT INTO orders (id, user_id, amount) VALUES
(1, 1, 100.00),
(2, 2, 200.00),
(3, 3, 300.00);
SELECT /*+ read_from_storage(tiflash[u]) */
u.name, o.amount
FROM users u
JOIN orders o ON u.id = o.user_id
where u.id=1;
(root@127.0.0.1) [test]>select * from information_schema.tiflash_replica;
+--------------+------------+----------+---------------+-----------------+-----------+----------+
| TABLE_SCHEMA | TABLE_NAME | TABLE_ID | REPLICA_COUNT | LOCATION_LABELS | AVAILABLE | PROGRESS |
+--------------+------------+----------+---------------+-----------------+-----------+----------+
| test | t11 | 111 | 1 | | 1 | 1 |
| test | users | 190 | 1 | | 1 | 1 |
+--------------+------------+----------+---------------+-----------------+-----------+----------+
2 rows in set (0.01 sec)
(root@127.0.0.1) [test]>set tidb_enforce_mpp=ON;
Query OK, 0 rows affected (0.00 sec)
(root@127.0.0.1) [test]>explain SELECT /*+ read_from_storage(tiflash[u]) */ u.name, o.amount FROM users u JOIN orders o ON u.id = o.user_id where u.id=1;
+--------------------------------------+---------+--------------+-------------------------------------+---------------------------------------------------------------------+
| id | estRows | task | access object | operator info |
+--------------------------------------+---------+--------------+-------------------------------------+---------------------------------------------------------------------+
| Projection_10 | 0.00 | root | | test.users.name, test.orders.amount |
| └─HashJoin_12 | 0.00 | root | | CARTESIAN inner join |
| ├─IndexLookUp_18(Build) | 0.00 | root | | |
| │ ├─IndexRangeScan_16(Build) | 0.00 | cop[tikv] | table:o, index:idx_user_id(user_id) | range:[1,1], keep order:false, stats:partial[user_id:unInitialized] |
| │ └─TableRowIDScan_17(Probe) | 0.00 | cop[tikv] | table:o | keep order:false, stats:partial[user_id:unInitialized] |
| └─TableReader_21(Probe) | 1.00 | root | | data:TableRangeScan_20 |
| └─TableRangeScan_20 | 1.00 | cop[tiflash] | table:u | range:[1,1], keep order:false, stats:pseudo |
+--------------------------------------+---------+--------------+-------------------------------------+---------------------------------------------------------------------+
7 rows in set, 1 warning (0.01 sec)
(root@127.0.0.1) [test]>show warnings;
+---------+------+----------------------------------------------------------------------------------+
| Level | Code | Message |
+---------+------+----------------------------------------------------------------------------------+
| Warning | 1105 | MPP mode may be blocked because there aren't tiflash replicas of table `orders`. |
+---------+------+----------------------------------------------------------------------------------+
1 row in set (0.00 sec)
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