Explain Statements Using JOIN
The SQL Optimizer needs to decide in which order tables should be joined and what is the most efficient join algorithm for a particular SQL statement.
Example
We have prepared a simple example to help you understand the execution plan for interpreting the JOIN using EXPLAIN.
> drop table if exists t1;
> create table t1 (id int,ti tinyint unsigned,si smallint,bi bigint unsigned,fl float,dl double,de decimal,ch char(20),vch varchar(20),dd date,dt datetime);
> insert into t1 values(1,1,4,3,1113.32,111332,1113.32,'hello','subquery','2022-04-28','2022-04-28 22:40:11');
> insert into t1 values(2,2,5,2,2252.05,225205,2252.05,'bye','sub query','2022-04-28','2022-04-28 22:40:11');
> insert into t1 values(3,6,6,3,3663.21,366321,3663.21,'hi','subquery','2022-04-28','2022-04-28 22:40:11');
> insert into t1 values(4,7,1,5,4715.22,471522,4715.22,'good morning','my subquery','2022-04-28','2022-04-28 22:40:11');
> insert into t1 values(5,1,2,6,51.26,5126,51.26,'byebye',' is subquery?','2022-04-28','2022-04-28 22:40:11');
> insert into t1 values(6,3,2,1,632.1,6321,632.11,'good night','maybe subquery','2022-04-28','2022-04-28 22:40:11');
> insert into t1 values(7,4,4,3,7443.11,744311,7443.11,'yes','subquery','2022-04-28','2022-04-28 22:40:11');
> insert into t1 values(8,7,5,8,8758.00,875800,8758.11,'nice to meet','just subquery','2022-04-28','2022-04-28 22:40:11');
> insert into t1 values(9,8,4,9,9849.312,9849312,9849.312,'see you','subquery','2022-04-28','2022-04-28 22:40:11');
> drop table if exists t2;
> create table t2 (id int,ti tinyint unsigned,si smallint,bi bigint unsigned,fl float,dl double,de decimal,ch char(20),vch varchar(20),dd date,dt datetime);
> insert into t2 values(1,1,4,3,1113.32,111332,1113.32,'hello','subquery','2022-04-28','2022-04-28 22:40:11');
> insert into t2 values(2,2,5,2,2252.05,225205,2252.05,'bye','sub query','2022-04-28','2022-04-28 22:40:11');
> insert into t2 values(3,6,6,3,3663.21,366321,3663.21,'hi','subquery','2022-04-28','2022-04-28 22:40:11');
> insert into t2 values(4,7,1,5,4715.22,471522,4715.22,'good morning','my subquery','2022-04-28','2022-04-28 22:40:11');
> insert into t2 values(5,1,2,6,51.26,5126,51.26,'byebye',' is subquery?','2022-04-28','2022-04-28 22:40:11');
> insert into t2 values(6,3,2,1,632.1,6321,632.11,'good night','maybe subquery','2022-04-28','2022-04-28 22:40:11');
> insert into t2 values(7,4,4,3,7443.11,744311,7443.11,'yes','subquery','2022-04-28','2022-04-28 22:40:11');
> insert into t2 values(8,7,5,8,8758.00,875800,8758.11,'nice to meet','just subquery','2022-04-28','2022-04-28 22:40:11');
> insert into t2 values(9,8,4,9,9849.312,9849312,9849.312,'see you','subquery','2022-04-28','2022-04-28 22:40:11');
Hash Join
In the Hash Join operation, MatrixOne first reads the smaller tables in t1 and t2 and uses a hash function for each value to be joined to obtain a hash table. Then each row of the other table is scanned, and the hash value is calculated and compared with the hash table generated in the previous step. A new join table is generated based on the joinif any values meet the join criteria.
The hash join operator is multi-threaded in MatrixOne and executes in parallel.
An example of hash join is as below:
> SELECT /*+ HASH_JOIN(t1, t2) */ * FROM t1, t2 WHERE t1.id = t2.id;
+------+------+------+------+----------+---------+------+--------------+----------------+------------+---------------------+------+------+------+------+----------+---------+------+--------------+----------------+------------+---------------------+
| id | ti | si | bi | fl | dl | de | ch | vch | dd | dt | id | ti | si | bi | fl | dl | de | ch | vch | dd | dt |
+------+------+------+------+----------+---------+------+--------------+----------------+------------+---------------------+------+------+------+------+----------+---------+------+--------------+----------------+------------+---------------------+
| 1 | 1 | 4 | 3 | 1113.32 | 111332 | 1113 | hello | subquery | 2022-04-28 | 2022-04-28 22:40:11 | 1 | 1 | 4 | 3 | 1113.32 | 111332 | 1113 | hello | subquery | 2022-04-28 | 2022-04-28 22:40:11 |
| 2 | 2 | 5 | 2 | 2252.05 | 225205 | 2252 | bye | sub query | 2022-04-28 | 2022-04-28 22:40:11 | 2 | 2 | 5 | 2 | 2252.05 | 225205 | 2252 | bye | sub query | 2022-04-28 | 2022-04-28 22:40:11 |
| 3 | 6 | 6 | 3 | 3663.21 | 366321 | 3663 | hi | subquery | 2022-04-28 | 2022-04-28 22:40:11 | 3 | 6 | 6 | 3 | 3663.21 | 366321 | 3663 | hi | subquery | 2022-04-28 | 2022-04-28 22:40:11 |
| 4 | 7 | 1 | 5 | 4715.22 | 471522 | 4715 | good morning | my subquery | 2022-04-28 | 2022-04-28 22:40:11 | 4 | 7 | 1 | 5 | 4715.22 | 471522 | 4715 | good morning | my subquery | 2022-04-28 | 2022-04-28 22:40:11 |
| 5 | 1 | 2 | 6 | 51.26 | 5126 | 51 | byebye | is subquery? | 2022-04-28 | 2022-04-28 22:40:11 | 5 | 1 | 2 | 6 | 51.26 | 5126 | 51 | byebye | is subquery? | 2022-04-28 | 2022-04-28 22:40:11 |
| 6 | 3 | 2 | 1 | 632.1 | 6321 | 632 | good night | maybe subquery | 2022-04-28 | 2022-04-28 22:40:11 | 6 | 3 | 2 | 1 | 632.1 | 6321 | 632 | good night | maybe subquery | 2022-04-28 | 2022-04-28 22:40:11 |
| 7 | 4 | 4 | 3 | 7443.11 | 744311 | 7443 | yes | subquery | 2022-04-28 | 2022-04-28 22:40:11 | 7 | 4 | 4 | 3 | 7443.11 | 744311 | 7443 | yes | subquery | 2022-04-28 | 2022-04-28 22:40:11 |
| 8 | 7 | 5 | 8 | 8758 | 875800 | 8758 | nice to meet | just subquery | 2022-04-28 | 2022-04-28 22:40:11 | 8 | 7 | 5 | 8 | 8758 | 875800 | 8758 | nice to meet | just subquery | 2022-04-28 | 2022-04-28 22:40:11 |
| 9 | 8 | 4 | 9 | 9849.312 | 9849312 | 9849 | see you | subquery | 2022-04-28 | 2022-04-28 22:40:11 | 9 | 8 | 4 | 9 | 9849.312 | 9849312 | 9849 | see you | subquery | 2022-04-28 | 2022-04-28 22:40:11 |
+------+------+------+------+----------+---------+------+--------------+----------------+------------+---------------------+------+------+------+------+----------+---------+------+--------------+----------------+------------+---------------------+
9 rows in set (0.00 sec)
mysql> EXPLAIN SELECT /*+ HASH_JOIN(t1, t2) */ * FROM t1, t2 WHERE t1.id = t2.id;
+------------------------------------+
| QUERY PLAN |
+------------------------------------+
| Project |
| -> Join |
| Join Type: INNER |
| Join Cond: (t1.id = t2.id) |
| -> Table Scan on db1.t1 |
| -> Table Scan on db1.t2 |
+------------------------------------+
6 rows in set (0.01 sec)
MatrixOne executes the Hash Join operator in the following order:
-
Scan table t2 and t1 in parallel.
-
Execute the
JOINfilter query:(t1.id = t2.id). -
Execute
INNER JOIN.