mysql - Why is this simple join query significantly quicker with a sub-query? -


i have 2 tables. order_details 100,000 rows, , outbound 10,000 rows.

i need join them on column called order_number, varchar(50) on both. order_number not unique in outbound table.

create table `outbound` (     `outbound_id` int(12) not null,     `order_number` varchar(50) not null ) engine=innodb default charset=utf8;  create table `order_details` (     `order_details_id` int(12) not null,     `order_number` varchar(50) not null ) engine=innodb default charset=utf8;

this initial query, , takes on 60 seconds run:

select o.order_number outbound o inner join order_details od     on o.order_number = od.order_number

this query gets same results , takes less second run:

select o.order_number outbound o inner join (     select order_number     order_details ) od on (o.order_number = od.order_number)

this surprising me because sub-queries slower.

running explain (which i'm still learning how understand) shows sub query version uses derived2 table, using index, , index auto_key0. i'm not savvy enough know how interpret understand why makes significant difference.

i running these queries on command line.

i running mysql ver 14.14 distrib 5.6.35, linux (x86_64) centos.

in summary:

why simple join query quicker sub-query?

my knowledge of mysql limited. these thoughts:

your tables don't have indexes. join has read entire second table in order compare, each row of first table.

the subquery reads second table once , creates index, doesn't need read entire second table each row of first table. has check index, more faster.

to verify if i'm ritght or not, try creating indexes column order_number in 2 tables (create index ... ), , run again 2 queries. first query should take less second instead of minute.


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