Mysql return table

Return form

Mysql return table refers to the index column queried by the secondary index under the InnoDB storage engine. If you need to find the data of all columns, you need to retrieve the data from the primary key index.This process is called backlisting.Because the data of the row is stored in the leaf nodes of the primary key B+tree, the B+ tree leaf nodes of the secondary index are stored (index columns, primary keys).

Example:

There is a user table id (primary key), username (secondary index), age

id is the primary key index username is the secondary index, when we return records through the secondary index query, for example:

select * from t where username = “bobo”;

① Find the bobo record and primary key id through the secondary index

2 Find the row record by going to the primary key index B+tree through the primary key id

There will be a problem here. When our user table has 150w records, sometimes the query does not go through the index, but scans the entire disk through All.Why is this?Let's briefly introduce the IO of full scan and index walk.

If the size of our row of data is 100 bytes, bobo has 50W rows of data. Suppose the B+tree height of the user table=3

If indexed by username: 3 IOs are required for each return to the table, and 50w records need to be returned to the table 50w times.Then IO = 50W * 3

Through full scan: one page can store 16k / 100 bytes = 16 * 1024 / 100 = 163 records, full scan requires 150w / 163 = 0.92W, which is almost 1w IO,

Through the above comparison, it must be more efficient overall, which is why sometimes we build an index, but our sql does not go into the index when it is detected by explain.Because the IO overhead of returning the table is still relatively large.

MRR

MRR simply means that the above return table is more efficient. If you need to return the table 50w times, the efficiency is too slow. At this time, Mysql sorts the primary key id that needs to be returned to the table, and then sorts the id after that.Go to the primary key B+ tree to fetch the database, which greatly increases the efficiency.We can judge whether MRR is used by the Using MRR in the Extra column in explain.

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