MySQL：创建临时表时是否自动创建主键？

Question

I have a query that is taking quite a long time (11 million or so observations), and three joins (I can't stop it to check). One of the joins is with a temporary table.

When I create a temporary table using data from a table that has a primary key within it, will the new table carry over the indexing, or do I have to explicitly create an index in the new temporary table (with the primary key from the parent table)?

Answer 1

No - For explicitly defined temporary tables no indexes are defined automatically. You will need to either define the index while the table creation or afterwards with ALTER TABLE .. .

You can check it with SHOW CREATE TABLE my_temptable .

Try the following script:

drop table if exists my_persisted_table;
create table my_persisted_table (
    id int auto_increment primary key,
    col varchar(50)
);
insert into my_persisted_table(col) values ('a'), ('b');

drop temporary table if exists my_temptable;
create temporary table my_temptable as 
    select * from my_persisted_table;

show create table my_temptable;

alter table my_temptable add index (id);

show create table my_temptable;

The first SHOW CREATE statement will show no indexes:

CREATE TEMPORARY TABLE `my_temptable` (
  `id` int(11) NOT NULL DEFAULT '0',
  `col` varchar(50) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8

After creating an index with ALTER TABLE we can see it with the second SHOW CREATE statement:

CREATE TEMPORARY TABLE `my_temptable` (
  `id` int(11) NOT NULL DEFAULT '0',
  `col` varchar(50) DEFAULT NULL,
  KEY `id` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

Demo: http://rextester.com/JZQCP29681

Answer 2

TEMPORARY tables have a very loose relationship with databases (schemas). Dropping a database does not automatically drop any TEMPORARY tables created within that database. Also, you can create a TEMPORARY table in a nonexistent database if you qualify the table name with the database name in the CREATE TABLE statement. In this case, all subsequent references to the table must be qualified with the database name.

during generation of TEMPORARY table you have to mention all record of the table

https://dev.mysql.com/doc/refman/5.7/en/create-temporary-table.html

Answer 3

This syntax also works:

create temporary table my_temptable
    ( PRIMARY KEY(id) )
    select * from my_persisted_table;

That is, you can have extra CREATE TABLE clauses in place from the start. This can be especially effective if the SELECT is delivering the rows to an InnoDB table in PK order:

create temporary table my_temptable
    ( PRIMARY KEY(id) )
        ENGINE=InnoDB
    select * from my_persisted_table
        ORDER BY id;