MySQL: Building the best INDEX for a given SELECT

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Brought to you by Rick James

The Problem

You have a SELECT and you want to build the best INDEX for it. This blog is a "cookbook" on how to do that task. 
    ⚈  A short algorithm that works for many simpler SELECTs and helps in complex queries. 
    ⚈  Examples of the algorithm, plus digressions into exceptions and variants. 
    ⚈  Finally a long list of "other cases".

⚈一种简短的算法，适用于许多更简单的SELECT，有助于复杂的查询。 
    ⚈算法示例，以及对异常和变体的深入研究。 
    ⚈最后列出了一长串“其他案例”。

The hope is that a newbie can quickly get up to speed, and his/her INDEXes will no longer smack of "newbie".

Many edge cases are explained, so even an expert may find something useful here.

Algorithm 算法

Here's the way to approach creating an INDEX, given a SELECT. Follow the steps below, gathering columns to put in the INDEX in order. When the steps give out, you usually have the 'perfect' index.

1.  Given a WHERE with a bunch of expressions connected by AND: Include the columns (if any), in any order, that are compared to a constant and not hidden in a function. 
    2.  You get one more chance to add to the INDEX; do the first of these that applies: 
    ⚈   2a. One column used in a 'range' -- BETWEEN, >, LIKE w/o leading wildcard, etc. 
    ⚈   2b. All columns, in order, of the GROUP BY. 
    ⚈   2c. All columns, in order, of the ORDER BY if there is no mixing of ASC and DESC.

在给定SELECT的情况下，这是创建INDEX的方法。按照以下步骤操作，收集列以按顺序放入INDEX。当步骤给出时，您通常会有“完美”索引。

1.给定WHERE，其中一组表达式通过AND连接：以任何顺序包含列（如果有），与常量进行比较而不隐藏在函数中。 
    2.您还有一次机会加入INDEX; 做第一个适用的： 
    ⚈2a。在'范围'中使用的一列 - BETWEEN，>，LIKE没有领先的通配符等 
    .⚈2b。GROUP BY的所有列按顺序排列。 
    ⚈2c。如果没有ASC和DESC的混合，则按顺序排列ORDER BY的所有列。

Digression

This blog assumes you know the basic idea behind having an INDEX. Here is a refresher on some of the key points.

Virtually all INDEXes in MySQL are structured as BTrees
BTrees allow very efficient for 
    ⚈  Given a key, find the corresponding row(s); 
    ⚈  "Range scans" -- That is start at one value for the key and repeatedly find the "next" (or "previous") row.

事实上，MySQL中的所有INDEX都被构造为 BTrees
BTrees允许非常高效 
    ⚈给定一个键，找到相应的行; 
    ⚈“范围扫描” - 从键的一个值开始并重复找到“下一个”（或“上一个”）行

A PRIMARY KEY is a UNIQUE KEY; a UNIQUE KEY is an INDEX. (KEY == INDEX.)

InnoDB "clusters" the PRIMARY KEY with the data. Hence, given the value of the PK ("PRIMARY KEY"), after drilling down the BTree to find the index entry, you have all the columns of the row when you get there. A "secondary key" (any UNIQUE or INDEX other than the PK) in InnoDB first drills down the BTree for the secondary index, where it finds a copy of the PK Then it drills down the PK to find the row.

Every InnoDB table has a PRIMARY KEY. While there is a default if you do not specify one, it is best to explicitly provide a PK.

For completeness: MyISAM works differently. All indexes (including the PK) are in separate BTrees. The leaf node of such BTrees have a pointer (usually a byte offset) into the data file.

All discussion here assumes InnoDB tables, however most statements apply to other Engines.

First, some examples

Think of a list of names, sorted by last_name, then first_name. You have undoubtedly seen such lists, and they often have other information such as address and phone number. Suppose you wanted to look me up. If you remember my full name ('James' and 'Rick'), it is easy to find my entry. If you remembered only my last name ('James') and first initial ('R'). You would quickly zoom in on the Jameses and find the Rs in them. There, you might remember 'Rick' and ignore 'Ronald'. But, suppose you remembered my first name ('Rick') and only my last initial ('J'). Now you are in trouble. You would be scanning all the Js -- Jones, Rick; Johnson, Rick; Jamison, Rick; etc, etc. That's much less efficient.

Those equate to

INDEX(last_name, first_name) -- the order of the list.
WHERE last_name = 'James' AND first_name = 'Rick' -- best case
WHERE last_name = 'James' AND first_name LIKE 'R%' -- pretty good
WHERE last_name LIKE 'J%' AND first_name = 'Rick' -- pretty bad

INDEX（last_name，first_name） - 列表的顺序。
WHERE last_name ='James'AND first_name ='Rick' - 最好的情况
WHERE last_name ='James'AND first_name LIKE'R％' - 相当不错
WHERE last_name LIKE'J％'和first_name ='Rick' - 非常糟糕

Think about this example as I talk about "=" ('equality predicate') versus "range" in the Algorithm, below. 
想想这个例子，我在下面的算法中谈到“=”（'等式谓词'）与“范围”。

Algorithm, Step 1 (WHERE "column = const")

⚈  WHERE aaa = 123 AND ... : an INDEX starting with aaa is good. 
    ⚈  WHERE aaa = 123 AND bbb = 456 AND ... : an INDEX starting with aaa and bbb is good. In this case, it does not matter whether aaa or bbb comes first in the INDEX. 
    ⚈  xxx IS NULL : this acts like "= const" for this discussion. 
    ⚈  WHERE t1.aa = 123 AND t2.bb = 456 -- You must only consider columns in the current table.

Note that the expression must be of the form of column_name = (constant). The following do not work for step 1: DATE(dt) = '...', LOWER(s) = '...', CAST(s ...) = '...', x='...' COLLATE... -- They "hide the column in a function call". The index cannot be used.

(If there are no "=" parts AND'd in the WHERE clause, move on to step 2 without any columns in your putative INDEX.)

Algorithm, Step 2

Find the first of 2a / 2b / 2c that applies; use it; then quit. If none apply, then you are through gathering columns for the index.

In some cases it is optimal to do step 1 (all equals) plus step 2c (ORDER BY).

Algorithm, Step 2a (one range)

A "range" shows up as 
    ⚈  aaa >= 123 -- any of <, <=, >=, >; but not <>, !=, IS NOT NULL 
    ⚈  aaa BETWEEN 22 AND 44 
    ⚈  sss LIKE 'blah%' -- but not sss LIKE '%blah' 
    ⚈  xxx IS NOT NULL 
Add the column in the range to your putative INDEX.

If there are more parts to the WHERE clause, you must stop now.

Complete examples (assume nothing else comes after the snippet) 
    ⚈  WHERE aaa >= 123 AND bbb = 1   ⇒ INDEX(bbb, aaa) (WHERE order does not matter; INDEX order does) 
    ⚈  WHERE aaa >= 123   ⇒ INDEX(aaa) 
    ⚈  WHERE aaa >= 123 AND ccc > 'xyz'   ⇒ INDEX(aaa) or INDEX(ccc) (only one range) 
    ⚈  WHERE aaa >= 123 ORDER BY aaa   ⇒ INDEX(aaa) -- Bonus: The ORDER BY will use the INDEX. 
    ⚈  WHERE aaa >= 123 ORDER BY aaa   ⇒ INDEX(aaa) DESC -- Same Bonus.

(There are some cases where multiple ranges on a single column will work.)

Algorithm, Step 2b (GROUP BY)

If there is a GROUP BY, all the columns of the GROUP BY should now be added, in the specified order, to the INDEX you are building. (I do not know what happens if one of the columns is already in the INDEX.)

If you are GROUPing BY an expression (including function calls), you cannot use the GROUP BY; stop.

Complete examples (assume nothing else comes after the snippet) 
    ⚈  WHERE aaa = 123 AND bbb = 1 GROUP BY ccc   ⇒ INDEX(bbb, aaa, ccc) or INDEX(aaa, bbb, ccc) (='s first, in any order; then the GROUP BY) 
    ⚈  WHERE aaa >= 123 GROUP BY xxx   ⇒ INDEX(aaa) (You should have stopped with Step 2a) 
    ⚈  GROUP BY x,y   ⇒ INDEX(x,y) (no WHERE) 
    ⚈  WHERE aaa = 123 GROUP BY xxx,(a+b)   ⇒ INDEX(aaa) -- expression in GROUP BY, so no use including even xxx.

Algorithm, Step 2c (ORDER BY)

If there is a ORDER BY, all the columns of the ORDER BY should now be added, in the specified order, to the INDEX you are building.

If there are multiple columns in the ORDER BY, and there is a mixture of ASC and DESC, do not add the ORDER BY columns; they won't help; stop. (There is an exception in version 8.0.)

If you are ORDERing BY an expression (including function calls), you cannot use the ORDER BY; stop.

Complete examples (assume nothing else comes after the snippet) 
    ⚈  WHERE aaa = 123 GROUP BY ccc ORDER BY ddd   ⇒ INDEX(aaa, ccc) -- should have stopped with Step 2b 
    ⚈  WHERE aaa = 123 GROUP BY ccc ORDER BY ccc   ⇒ INDEX(aaa, ccc) -- the ccc will be used for both GROUP BY and ORDER BY 
    ⚈  WHERE aaa = 123 ORDER BY xxx ASC, yyy DESC   ⇒ INDEX(aaa) -- mixture of ASC and DESC.

The following are especially good. Normally a LIMIT cannot be applied until after lots of rows are gathered and then sorted according to the ORDER BY. But, if the INDEX gets all they way through the ORDER BY, only (OFFSET + LIMIT) rows need to be gathered. So, in these cases, you win the lottery with your new index: 
    ⚈  WHERE aaa = 123 GROUP BY ccc ORDER BY ccc LIMIT 10   ⇒ INDEX(aaa, ccc) 
    ⚈  WHERE aaa = 123 ORDER BY ccc LIMIT 10   ⇒ INDEX(aaa, ccc) 
    ⚈  ORDER BY ccc LIMIT 10   ⇒ INDEX(ccc) 
    ⚈  WHERE ccc > 432 ORDER BY ccc LIMIT 10   ⇒ INDEX(ccc) -- This "range" is compatible with ORDER BY 
    ⚈  WHERE ccc < 432 ORDER BY ccc LIMIT 10   ⇒ INDEX(ccc) -- also works

(It does not make much sense to have a LIMIT without an ORDER BY, so I do not discuss that case.)

Algorithm end

You have collected a few columns; put them in INDEX and ADDed that to the table. That will often produce a "good" index for the SELECT you have. Below are some other suggestions that may be relevant.

An example of the Algorithm being 'wrong':

SELECT ... FROM t WHERE flag = true;

This would (according to the Algorithm) call for INDEX(flag). However, indexing a column that has two (or a small number of) values is almost always useless. This is called 'low cardinality'. The Optimizer would prefer to do a table scan than to bounce between the index BTree and the data.

On the other hand, the Algorithm is 'right' with

SELECT ... FROM t WHERE flag = true AND date >= '2015-01-01';

That would call for a compound index starting with a flag: INDEX(flag, date). Such an index is likely to be very beneficial. And it is likely to be more beneficial than INDEX(date).

Even more striking is with a LIMIT:

SELECT ... FROM t WHERE flag = true AND date >= '2015-01-01' LIMIT 10;

INDEX(flag, date) would be able to stop after 10 rows; INDEX(date) would not.

If your resulting INDEX include column(s) that are likely to be UPDATEd, note that the UPDATE will have extra work to remove a 'row' from one place in the INDEX's BTree and insert a 'row' back into the BTree. For example:

INDEX(x)
UPDATE t SET x = ... WHERE ...;

There are too many variables to say whether it is better to keep the index or to toss it.

Limitations 限制

(There are exceptions to some of these.) 
    ⚈  In 5.6 / 10.0, you may not include a column that equates to bigger than 767 bytes: VARCHAR(255) CHARACTER SET utf8 or VARCHAR(191) CHARACTER SET utf8mb4. 
    ⚈  In 5.7 / 10.2, you may not include a column that equates to bigger than 3072 bytes. 
    ⚈  You can deal with big fields using "prefix" indexing; but see below. 
    ⚈  You should not have more than 5 columns in an index. (This is just a Rule of Thumb; up to 16 is allowed.) 
    ⚈  You should not have redundant indexes. (See below.)

（其中一些有例外。） 
    ⚈在5.6 / 10.0中，您可能不包括等于大于767字节的列：VARCHAR（255）CHARACTER SET utf8或VARCHAR（191）CHARACTER SET utf8mb4。 
    ⚈在5.7 / 10.2中，您可能不包含等于大于3072字节的列。 
    ⚈您可以使用“前缀”索引来处理大字段; 但见下文。 
    ⚈索引中的列不应超过5列。（这只是一个经验法则;允许最多16个。） 
    ⚈你不应该有冗余索引。（见下文。）

Stop after equality test

Technically, it is more complex than testing for '=': The relevant part from the documentation: "The optimizer attempts to use additional key parts to determine the interval as long as the comparison operator is =, <=>, or IS NULL. If the operator is >, <, >=, <=, !=, <>, BETWEEN, or LIKE, the optimizer uses it but considers no more key parts." See Range Optimization

Flags and Low Cardinality

INDEX(flag) is almost never useful if flag has very few values. More specifically, when you say WHERE flag = 1 and "1" occurs more than 20% of the time, such an index will be shunned. The Optimizer would prefer to scan the table instead of bouncing back and forth between the index and the data for more than 20% of the rows.

("20%" is really somewhere between 10% and 30%, depending on the phase of the moon.)

Example of 20% cutoff

Discussions about Cardinality: Cardinality & Range
Cardinality & composite
Single-column index

关于基数的讨论： 基数和范围
基数和复合
单列索引

"Covering" Indexes “覆盖”索引

A "Covering" index is an index that contains all the columns in the SELECT. It is special in that the SELECT can be completed by looking only at the INDEX BTree. (Since InnoDB's PRIMARY KEY is clustered with the data, "covering" is of no benefit when considering at the PRIMARY KEY.)

“覆盖”索引是包含SELECT 中所有列的索引。特别之处在于，只需查看INDEX BTree即可完成SELECT。（由于InnoDB的PRIMARY KEY与数据聚集在一起，因此在覆盖PRIMARY KEY时“覆盖”没有任何好处。）

Mini-cookbook: 
    1.  Gather the list of column(s) according to the "Algorithm", above. 
    2.  Add to the end of the list the rest of the columns seen in the SELECT, in any order.

Examples: 
    ⚈  SELECT x FROM tbl WHERE y = 5;   ⇒ INDEX(y,x) -- The algorithm said just INDEX(y) 
    ⚈  SELECT x,z FROM tbl WHERE y = 5 AND q = 7;   ⇒ INDEX(y,q,x,z) -- y and q in either order (Algorithm), then x and z in either order (covering). 
    ⚈  SELECT x FROM tbl WHERE y > 5 AND q > 7;   ⇒ INDEX(y,q,x) -- y or q first (that's as far as the Algorithm goes), then the other two fields afterwards.

The speedup you get might be minor, or it might be spectacular; it is hard to predict.

But... 
    ⚈  It is not wise to build an index with lots of columns. Let's cut it off at 5 (Rule of Thumb). 
    ⚈  Prefix indexes cannot 'cover', so don't use them anywhere in a 'covering' index. 
    ⚈  There are limits (3KB?) on how 'wide' an index can be, so "covering" may not be possible.

但是...... 
    build建立一个包含大量列的索引是不明智的。让我们在5（经验法则）切断它。 
    ⚈前缀索引不能“覆盖”，因此不要在“覆盖”索引中的任何位置使用它们。 
    ⚈索引的“宽度”有限制（3KB？），因此可能无法“覆盖”。

Redundant/Excessive Indexes

INDEX(a,b) can find anything that INDEX(a) could find. So you don't need both. Get rid of the shorter one.

If you have lots of SELECTs and they generate lots of INDEXes, this may cause a different problem. Each index must be updated (sooner or later) for each INSERT. More indexes   ⇒ slower INSERTs. Limit the number of indexes on a table to about 6 (Rule of Thumb).

Notice in the cookbook how it says "in any order" in a few places. If, for example, you have both of these (in different SELECTs): 
    ⚈  WHERE a=1 AND b=2 begs for either INDEX(a,b) or INDEX(b,a) 
    ⚈  WHERE a>1 AND b=2 begs only for INDEX(b,a) 
Include only INDEX(b,a) since it handles both cases with only one INDEX.

Suppose you have a lot of indexes, including (a,b,c,dd) and (a,b,c,ee). Those are getting rather long. Consider either picking one of them, or having simply (a,b,c). Sometimes the selectivity of (a,b,c) is so good that tacking on 'dd' or 'ee' does make enough difference to matter.

Optimizer picks ORDER BY

The main cookbook skips over an important optimization that is sometimes used. The optimizer will sometimes ignore the WHERE and, instead, use an INDEX that matches the ORDER BY. This, of course, needs to be a perfect match -- all columns, in the same order. And all ASC or all DESC.

This becomes especially beneficial if there is a LIMIT.

But there is a problem. There could be two situations, and the Optimizer is sometimes not smart enough to see which case applies: 
    ⚈  If the WHERE does very little filtering, fetching the rows in ORDER BY order avoids a sort and has little wasted effort (because of 'little filtering'). Using the INDEX matching the ORDER BY is better in this case. 
    ⚈  If the WHERE does a lot of filtering, the ORDER BY is wasting a lot of time fetching rows only to filter them out. Using an INDEX matching the WHERE clause is better.

What should you do? If you think the "little filtering" is likely, then create an index with the ORDER BY columns in order and hope that the Optimizer uses it when it should.

OR

Cases... 
    ⚈  WHERE a=1 OR a=2 -- This is turned into WHERE a IN (1,2) and optimized that way. 
    ⚈  WHERE a=1 OR b=2 usually cannot be optimized. 
    ⚈  WHERE x.a=1 OR y.b=2 This is even worse because of using two different tables.

A workaround is to use UNION. Each part of the UNION is optimized separately. For the second case:

( SELECT ... WHERE a=1 ) -- and have INDEX(a)
UNION DISTINCT -- "DISTINCT" is assuming you need to get rid of dups
( SELECT ... WHERE b=2 ) -- and have INDEX(b)
GROUP BY ... ORDER BY ... -- whatever you had at the end of the original query

Now the query can take good advantage of two different indexes. Note: "Index merge" might kick in on the original query, but it is not necessarily any faster than the UNION. Sister blog on compound indexes, including 'Index Merge'

The third case (OR across 2 tables) is similar to the second.

If you originally had a LIMIT, UNION gets complicated. If you started with ORDER BY z LIMIT 190, 10, then the UNION needs to be

( SELECT ... LIMIT 200 ) -- Note: OFFSET 0, LIMIT 190+10
UNION DISTINCT -- (or ALL)
( SELECT ... LIMIT 200 )
LIMIT 190, 10 -- Same as originally

TEXT / BLOB

You cannot directly index a TEXT or BLOB or large VARCHAR or large BINARY column. However, you can use a "prefix" index: INDEX(foo(20)). This says to index the first 20 characters of foo. But... It is rarely useful. 
您不能直接索引TEXT或BLOB或大型VARCHAR或大型BINARY列。但是，您可以使用“前缀”索引： INDEX（foo（20））。这表示索引foo的前20个字符。但是......它很少有用。

Example of a prefix index:

INDEX(last_name(2), first_name)

The index for me would contain 'Ja', 'Rick'. That's not useful for distinguishing between 'Jamison', 'Jackson', 'James', etc., so the index is so close to useless that the optimizer often ignores it.

Probably never do UNIQUE(foo(20)) because this applies a uniqueness constraint on the first 20 characters of the column, not the whole column!

More on prefix indexing 有关前缀索引的更多信息

DATEs

DATE, DATETIME, etc. are tricky to compare against.

Some tempting, but inefficient, techniques:

date_col LIKE '2016-01%' -- must convert date_col to a string, so acts like a function
LEFT(date_col, 4) = '2016-01' -- hiding the column in function
DATE(date_col) = 2016 -- hiding the column in function
一些诱人但效率低下的技术：

date_col LIKE'2016-01％' - 必须将date_col转换为字符串，因此就像一个函数
LEFT（date_col，4）='2016-01' - 将列隐藏在函数中
DATE（date_col）= 2016 - 将列隐藏在函数中

All must do a full scan. (On the other hand, it can handy to use GROUP BY LEFT(date_col, 7) for monthly grouping, but that is not an INDEX issue.) 
都必须进行全面扫描。（另一方面，它可以方便地使用GROUP BY LEFT（date_col，7）进行月度分组，但这不是INDEX问题。）

This is efficient, and can use an index:

date_col >= '2016-01-01'
AND date_col < '2016-01-01' + INTERVAL 3 MONTH

This case works because both right-hand values are converted to constants, then it is a "range". I like the design pattern with INTERVAL because it avoids computing the last day of the month. And it avoids tacking on '23:59:59', which is wrong if you have microsecond times. (And other cases.)

EXPLAIN Key_len

Perform EXPLAIN SELECT... (and EXPLAIN FORMAT=JSON SELECT... if you have 5.6.5). Look at the Key that it chose, and the Key_len. From those you can deduce how many columns of the index are being used for filtering. (JSON makes it easier to get the answer.) From that you can decide whether it is using as much of the INDEX as you thought. Caveat: Key_len only covers the WHERE part of the action; the non-JSON output won't easily say whether GROUP BY or ORDER BY was handled by the index.

执行EXPLAIN SELECT ...（和EXPLAIN FORMAT = JSON SELECT ...如果你有5.6.5）。看看它选择的Key和Key_len。从中可以推断出索引的列数用于过滤。（JSON可以更容易地得到答案。）从中您可以决定它是否正在使用尽可能多的INDEX。警告：Key_len仅涵盖动作的WHERE部分; 非JSON输出不会轻易说明索引是否处理GROUP BY或ORDER BY。

IN

IN (1,99,3) is sometimes optimized as efficiently as "=", but not always. Older versions of MySQL did not optimize it as well as newer versions. (5.6 is possibly the main turning point.)

IN ( SELECT ... )

From version 4.1 through 5.5, IN ( SELECT ... ) was very poorly optimized. The SELECT was effectively re-evaluated every time. Often it can be transformed into a JOIN, which works much faster. Heres is a pattern to follow:

SELECT ...
FROM a
WHERE test_a
AND x IN (
SELECT x
FROM b
WHERE test_b
);
⇒
SELECT ...
FROM a
JOIN b USING(x)
WHERE test_a
AND test_b;

The SELECT expressions will need "a." prefixing the column names.

Alas, there are cases where the pattern is hard to follow.

5.6 does some optimizing, but probably not as good as the JOIN.

If there is a JOIN or GROUP BY or ORDER BY LIMIT in the subquery, that complicates the JOIN in new format. So, it might be better to use this pattern:

SELECT ...
FROM a
WHERE test_a
AND x IN ( SELECT x FROM ... );
⇒
SELECT ...
FROM a
JOIN ( SELECT x FROM ... ) b
USING(x)
WHERE test_a;

Caveat: If you end up with two subqueries JOINed together, note that neither has any indexes, hence performance can be very bad. (5.6 improves on it by dynamically creating indexes for subqueries.)

There is work going on in MariaDB and Oracle 5.7, in relation to "NOT IN", "NOT EXISTS", and "LEFT JOIN..IS NULL"; here is an old discussion on the topic
So, what I say here may not be the final word.

Explode/Implode

When you have a JOIN and a GROUP BY, you may have the situation where the JOIN exploded more rows than the original query (due to many:many), but you wanted only one row from the original table, so you added the GROUP BY to implode back to the desired set of rows.

This explode + implode, itself, is costly. It would be better to avoid them if possible.

Sometimes the following will work.

Using DISTINCT or GROUP BY to counteract the explosion

SELECT DISTINCT
a.*,
b.y
FROM a
JOIN b
⇒
SELECT a.*,
( SELECT GROUP_CONCAT(b.y) FROM b WHERE b.x = a.x ) AS ys
FROM a

When using second table just to check for existence:

SELECT a.*
FROM a
JOIN b ON b.x = a.x
GROUP BY a.id
⇒
SELECT a.*,
FROM a
WHERE EXISTS ( SELECT * FROM b WHERE b.x = a.x )

Another variant

Many-to-Many Mapping table 多对多映射表

Do it this way.

CREATE TABLE XtoY (
# No surrogate id for this table
x_id MEDIUMINT UNSIGNED NOT NULL, -- For JOINing to one table
y_id MEDIUMINT UNSIGNED NOT NULL, -- For JOINing to the other table
# Include other fields specific to the 'relation'
PRIMARY KEY(x_id, y_id), -- When starting with X
INDEX (y_id, x_id) -- When starting with Y
) ENGINE=InnoDB;

Notes: 
    ⚈  Lack of an AUTO_INCREMENT id for this table -- The PK given is the 'natural' PK; there is no good reason for a surrogate. 
    ⚈  "MEDIUMINT" -- This is a reminder that all INTs should be made as small as is safe (smaller ⇒ faster). Of course the declaration here must match the definition in the table being linked to. 
    ⚈  "UNSIGNED" -- Nearly all INTs may as well be declared non-negative 
    ⚈  "NOT NULL" -- Well, that's true, isn't it? 
    ⚈  "InnoDB" -- More effecient than MyISAM because of the way the PRIMARY KEY is clustered with the data in InnoDB. 
    ⚈  "INDEX(y_id, x_id)" -- The PRIMARY KEY makes it efficient to go one direction; this index makes the other direction efficient. No need to say UNIQUE; that would be extra effort on INSERTs. 
    ⚈  In the secondary index, saying just INDEX(y_id) would work because it would implicit include x_id. But I would rather make it more obvious that I am hoping for a 'covering' index.

To conditionally INSERT new links, use IODKU

Note that if you had an AUTO_INCREMENT in this table, IODKU would "burn" ids quite rapidly.

Subqueries and UNIONs

Each subquery SELECT and each SELECT in a UNION can be considered separately for finding the optimal INDEX. 
可以单独考虑每个子查询SELECT和UNION中的每个SELECT以查找最佳INDEX。

Exception: In a "correlated" ("dependent") subquery, the part of the WHERE that depends on the outside table is not easily factored into the INDEX generation. (Cop out!)

JOINs

The first step is to decide what order the optimizer will go through the tables. If you cannot figure it out, then you may need to be pessimistic and create two indexes for each table -- one assuming the table will be used first, one assiming that it will come later in the table order.

The optimizer usually starts with one table and extracts the data needed from it. As it finds a useful (that is, matches the WHERE clause, if any) row, it reaches into the 'next' table. This is called NLJ ("Nested Loop Join"). The process of filtering and reaching to the next table continues through the rest of the tables.

The optimizer usually picks the "first" table based on these hints: 
    ⚈  STRAIGHT_JOIN forces the the table order. -STRAIGHT_JOIN强制表顺序。
    ⚈  The WHERE clause limits which rows needed (whether indexed or not). 
    ⚈  The table to the "left" in a LEFT JOIN usually comes before the "right" table. (By looking at the table definitions, the optimizer may decide that "LEFT" is irrelevant.) 
    ⚈  The current INDEXes will encourage an order. 
    ⚈  etc.

Running EXPLAIN tells you the table order that the Optimizer is very likely to use today. After adding a new INDEX, the optimizer may pick a different table order. You should anticipate the order changing, guess at what order makes the most sense, and build the INDEXes accordingly. Then rerun EXPLAIN to see if the Optimizer's brain was on the same wavelength you were on.

You should build the INDEX for the "first" table based on any parts of the WHERE, GROUP BY, and ORDER BY clauses that are relevant to it. If a GROUP/ORDER BY mentions a different table, you should ignore that clause.

The second (and subsequent) table will be reached into based on the ON clause. (Instead of using commajoin, please write JOINs with the JOIN keyword and ON clause!) In addition, there could be parts of the WHERE clause that are relevant. GROUP/ORDER BY are not to be considered in writing the optimal INDEX for subsequent tables.

PARTITIONing

PARTITIONing is rarely a substitute for a good INDEX.

PARTITION BY RANGE is a technique that is sometimes useful when indexing fails to be good enough. In a two-dimensional situation such as nearness in a geographical sense, one dimension can partially be handled by partition pruning; then the other dimension can be handled by a regular index (preferrably the PRIMARY KEY). This goes into more detail: Find nearest 10 pizza parlors

FULLTEXT

FULLTEXT is now implemented in InnoDB as well as MyISAM. It provides a way to search for "words" in TEXT columns. This is much faster (when it is applicable) than col LIKE '%word%'.

WHERE x = 1
AND MATCH (...) AGAINST (...)

always(?) uses the FULLTEXT index first. That is, the whole Algorithm is invalidated when one of the ANDs is a MATCH.

Signs of a Newbie 新手的迹象

⚈  No "compound" (aka "composite") indexes 
    ⚈  No PRIMARY KEY 
    ⚈  Redundant indexes (especially blatant is PRIMARY KEY(id), KEY(id)) 
    ⚈  Most or all columns individually indexes ("But I indexed everything") 
    ⚈  "Commajoin" -- That is FROM a, b WHERE a.x=b.x instead of FROM a JOIN b ON a.x=b.x

Speeding up wp_postmeta

The published table (see Wikipedia) is

CREATE TABLE wp_postmeta (
meta_id bigint(20) unsigned NOT NULL AUTO_INCREMENT,
post_id bigint(20) unsigned NOT NULL DEFAULT '0',
meta_key varchar(255) DEFAULT NULL,
meta_value longtext,
PRIMARY KEY (meta_id),
KEY post_id (post_id),
KEY meta_key (meta_key)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

The problems:

⚈  The AUTO_INCREMENT provides no benefit; in fact it slows down most queries (because of having to look in secondary index to find auto_inc id, then looking in data for actual id you need) 
    ⚈  The AUTO_INCREMENT is extra clutter - both on disk and in cache. 
    ⚈  Much better is PRIMARY KEY(post_id, meta_key) -- clustered, handles both parts of usual JOIN. 
    ⚈  BIGINT is overkill, but that can't be fixed without changing other tables. 
    ⚈  VARCHAR(255) can be a problem in 5.6 with utf8mb4; see workarounds below. 
    ⚈  When would meta_key or meta_value ever be NULL?

AUTOAUTO_INCREMENT没有任何好处; 实际上它会减慢大多数查询的速度（因为必须查看二级索引才能找到auto_inc id，然后在数据中查找您需要的实际ID） 
    ⚈AUTO_INCREMENT是额外的混乱 - 无论是在磁盘上还是在缓存中。 
    ⚈更好的是PRIMARY KEY（post_id，meta_key） - clustered，处理通常JOIN的两个部分。 
    ⚈BIGINT过度杀伤，但如果不改变其他表格则无法修复。 
    ⚈VARCHAR（255）可能是5.6中的问题与utf8mb4; 请参阅下面的解决方法。 
    when meta_key或meta_value何时为NULL？

The solutions:

CREATE TABLE wp_postmeta (
post_id BIGINT UNSIGNED NOT NULL,
meta_key VARCHAR(255) NOT NULL,
meta_value LONGTEXT NOT NULL,
PRIMARY KEY(post_id, meta_key),
INDEX(meta_key)
) ENGINE=InnoDB;

The typical usage:

JOIN wp_postmeta AS m ON p.id = m.post_id
WHERE m.meta_key = '...'

⚈  The composite PRIMARY KEY goes straight to the desired row, no digression through secondary index, nor search through multiple rows. 
    ⚈  INDEX(meta_key) may or may not be useful, depending on what other queries you have. 
    ⚈  InnoDB is required for the 'clustering'. 
    ⚈  Going forward, use utf8mb4, not utf8. However, you should be consistent across all WP tables and in your connection parameters.

The error "max key length is 767", which can happen in MySQL 5.6 when trying to use CHARACTER SET utf8mb4. Do one of the following (each has a drawback) to avoid the error:

⚈  Upgrade to 5.7.7 for 3072 byte limit -- your cloud may not provide this; 
    ⚈  Change 255 to 191 on the VARCHAR -- you lose any keys longer than 191 characters (unlikely?); 
    ⚈  ALTER .. CONVERT TO utf8 -- you lose Emoji and some of Chinese; 
    ⚈  Use a "prefix" index -- you lose some of the performance benefits; 
    ⚈  Reconfigure (if staying with 5.6.3 - 5.7.6) -- 4 things to change: Barracuda + innodb_file_per_table + innodb_large_prefix + dynamic or compressed.

错误“最大密钥长度为767”，这在MySQL 5.6中尝试使用CHARACTER SET utf8mb4时会发生。执行以下操作之一（每个都有一个缺点）以避免错误：

⚈升级到5.7.7以获得3072字节限制 - 您的云可能无法提供此功能; 
    ⚈在VARCHAR上将255更改为191 - 您丢失的长度超过191个字符（不太可能？）; 
    ⚈ALTER .. CONVERT TO utf8 - 你失去了表情符号和一些中文; 
    ⚈使用“前缀”索引 - 您将失去一些性能优势; 
    ⚈重新配置（如果保持5.6.3 - 5.7.6） - 要改变的4件事：Barracuda + innodb_file_per_table + innodb_large_prefix +动态或压缩。

If you need the ability to have multiple meta keys with the same key name for one post, then use this solution. It is nearly as good as the above usggestion.

CREATE TABLE wp_postmeta (
meta_id BIGINT UNSIGNED NOT NULL AUTO_INCREMENT,
post_id BIGINT UNSIGNED NOT NULL,
meta_key VARCHAR(255) NOT NULL,
meta_value LONGTEXT NOT NULL,
PRIMARY KEY(post_id, meta_key, meta_id), -- to allow dup meta_key for a post
INDEX(meta_id), -- to keep AUTO_INCREMENT happy
INDEX(meta_key)
) ENGINE=InnoDB;

StackOverflow
WordPress forum - on slow postmeta

Consider changing wp_commentmeta, wp_sitemeta, and wp_usermeta in similar ways.

Postlog

Initial posting: March, 2015;   Refreshed Feb, 2016;   Add DATE June, 2016;   WP example May and Sep, 2017

The tips in this document apply to MySQL, MariaDB, and Percona.

Good slide deck on BTrees

Percona 2015 Tutorial Slides

Sheeri Cabral's Index PDF
Sheeri Cabral's Index talk
Sheeri's EXPLAIN talk

Bill Karwin on designing indexes

Some info in the manual: ORDER BY Optimization
A short, but complicated, example

Manual page on range accesses in composite indexes

Some discussion of JOIN

Detecting whether an index is needed

This blog is the consolidation of a Percona tutorial I gave in 2013, plus many years of experience in fixing thousands of slow queries on hundreds of systems.

I appologize that this does not tell you how create INDEXes for all SELECTs. Some are just too complex.

Indexing 101: Optimizing MySQL queries on a single table
(Stephane Combaudon - Percona)

A complex query, well explained.

[[https://www.percona.com/blog/2009/03/05/what-does-using-filesort-mean-in-mysql/][What does Using filesort mean in MySQL?]

JOIN - with step-by-step explanation

Do not start index with a 'range'

Natural PK vs AUTO_INCREMENT debate

-- Rick James