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MySQL Index

2016/04/20

Quick note about MySQL Index.

Why Index

Indexes are used to find rows with specific column values quickly. Without an index, MySQL must begin with the first row and then read through the entire table to find the relevant rows. The larger the table, the more this costs. If the table has an index for the columns in question, MySQL can quickly determine the position to seek to in the middle of the data file without having to look at all the data. This is much faster than reading every row sequentially.

Most MySQL indexes are stored in B-trees. But indexes on spatial data types use R-trees;

Index Type

  1. INDEX. Duplicate values or composites are allowed.
  2. Composite Index. INDEX with multiple-column.
  3. UNIQUE INDEX. Duplicate values or composites are not allowed.
  4. PRIMARY KEY. Special unique index. Only one in each table.

Composite Index

Composite Index is multiple-column index. For example:

ALTER TABLE tb_name ADD INDEX name_city_age (name(10), city, age);

This composite index, in fact, is equivalent to the following three indexes:

INDEX (name, city, age) 
INDEX (name, city)
INDEX (name)

But NO indexes like:

INDEX (name, age) 
INDEX (city, age)

Because of leftmost prefix. It could simply be considered as composite from left. So the following select will benefit from this composite index:

SELECT * FROM tb_name WHREE name="admin" AND city="Paris" 
SELECT * FROM tb_name WHREE name="admin"

The following will not use it：

SELECT * FROM tb_name WHREE age=20 AND city="Paris" 
SELECT * FROM tb_name WHREE city="Paris"

Unique Index

A unique index enforces the constraint that we cannot have two equal values in that column. When unique with composite index, no two equal composites are allowed.

For example:

CREATE TABLE table1 (foo INT, bar INT);

CREATE UNIQUE INDEX ux_table1_foo ON table1(foo);  -- Create unique index on foo.

INSERT INTO table1 (foo, bar) VALUES (1, 2); -- OK
INSERT INTO table1 (foo, bar) VALUES (2, 2); -- OK
INSERT INTO table1 (foo, bar) VALUES (3, 1); -- OK
INSERT INTO table1 (foo, bar) VALUES (1, 4); -- Fails! Duplicate entry '1' for key 'ux_table1_foo'

The last insert fails because it violates the unique index on column foo when it tries to insert the value 1 into this column for a second time.

In MySQL, UNIQUE constraint allows multiple NULLs.

PRIMARY KEY

PRIMARY KEY is a special situation of UNIQUE

But some points different:

  • PRIMARY KEY implies NOT NULL, but unique index can be nullable.
  • There can be only one PRIMARY KEY, but there can be multiple unique indexes.
  • If there is no clustered index defined then PRIMARY KEY will be the clustered index.

Create Index

Three ways to create index

  1. CREATE TABLE
  2. ALTER TABLE
  3. CREATE INDEX (Not allowed to create PRIMARY KEY)
CREATE TABLE tb_name(  
  ...
  INDEX indexName (column_list),
  UNIQUE INDEX index_name (column_list),
  PRIMARY KEY(ID) 
); 

ALTER TABLE table_name ADD INDEX index_name (column_list);
ALTER TABLE table_name ADD UNIQUE INDEX index_name (column_list);
ALTER TABLE table_name ADD PRIMARY KEY (column_list);

CREATE INDEX index_name ON table_name (column_list);
CREATE UNIQUE INDEX index_name ON table_name (column_list);

Delete Index

DROP INDEX index_name ON talbe_name
ALTER TABLE table_name DROP INDEX index_name

# To delete primary key, as it iss unique in each table, no need to indicate index name
ALTER TABLE table_name DROP PRIMARY KEY

Delete a column from a table, index will also be influenced. For composite index, if one column inside the index is removed, the composite index will also remove it from its compostes. What’s more, if all columns from this index are removed, the composite index will be removed entirely.

Show index

mysql> show index from tb_name;
mysql> show keys from tb_name;

Disadvantage

Adding index increases query speed. But:

  • It decreases also speed to update, insert or delete.
  • Mysql comsumes more resources to maintain indexes
  • Indxes files occupy more spaces on disk

So we need to know how to choose columns to set index and also optimize queries.

How to choose columns for index

Consider about columns as conditions in queries frequently called.
Consider about columns in WHERE and JOIN.
SELECT t.Name 
FROM table1 t 
LEFT JOIN table2 m
ON t.name = m.username 
WHERE m.age=20 AND m.city='Paris'

So we could consider about setting index for age, city in WHERE or username in JOIN. Only one index can be used per SELECT/statement in the query

Index works with these operators <,<=,=,>,>=,between,in and like (Only for expression not beginning with% or _).

Not suggest columns which will be updated, inserted or deleted too frequently.
Better to choose columns with big cardinality, so different values. It is easy to distinguish birthday with differentdates. It means nothing set index on gendar to distinguish gendar with only ‘M’ or ‘F’. Because always half will be left.

We could compte Index Selectivity to help us to choose.

Index Selectivity =  Cardinality / Rows of table

The higher the index selectivity value the more suggested to choose.

In composite index, consider about leftmost, column with big index selectivity should be on left.
Consider table with many records, for example, 2000 records.
Consider short index to reduce size of index. Sometimes we don’t need to index on the entire field, we could set a prefix length. For exampe, a column is CHAR(200), if in the first 10 characters, most records are unique.We could use short index:
ALTER TABLE tb_name ADD INDEX index_name (long_string(10));

So mysql only indexes according to first 10 characters which involves less disk I/O, comsumes less index spaces, so may increase query speed.

Do not!

  • Index does not work with <>,not in, !=.

  • Do not calculate on columns, it will make index invalid.

select * from users where YEAR(adddate) < 2007;

Better change to like this:

select * from users where adddate < '2007-01-01';
  • Avoid to use NULL as default value for indexed column.

Refs

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