MySQL understand Indexes

Understanding Indexing
Without Needing to Understand Data Structures

Boston MySQL Meetup - March 14, 2011

Zardosht Kasheff

What’s a Table?
A dictionary is a set of (key, value) pairs.
• We’ll assume you can update the dictionary (insertions,
deletions, updates) and query the dictionary (point
queries, range queries)
• B-Trees and Fractal Trees are examples of dictionaries
• Hashes are not (range queries are not supported)

2

What’s a Table?

A table is a set of
a b c
dictionaries.
100 5 45
Example: 101 92 2
create table foo (a 156 56 45
int, b int, c int, 165 6 2
primary key(a)); 198 202 56
206 23 252
Then we insert a bunch 256 56 2
of data and get... 412 43 45

3

What’s a Table?
The sort order of a dictionary is deﬁned by the key
• For the data structures/storage engines we’ll think about,
range queries on the sort order are FAST
• Range queries on any other order require a table scan =
SLOW
• Point queries -- retrieving the value for one particular key --
is SLOW
‣ A single point query is fast, but reading a bunch of rows this way is going
to be 2 orders of magnitude slower than reading the same number of
rows in range query order

4

What’s an Index?
A Index I on table T is itself a dictionary
• We need to define the (key, value) pairs.
• The key in index I is a subset of fields in the primary
dictionary T.
• The value in index I is the primary key in T.
‣ There are other ways to define the value, but we’re sticking with this.

Example:
alter table foo add key(b);
Then we get...

5

What’s an Index?

Primary key(b)
a b c b a
100 5 45 5 100
101 92 2 6 165
156 56 45 23 206
165 6 2 43 412
198 202 56 56 156
206 23 252 56 256
256 56 2 92 101
412 43 45 202 198

6

Q: count(*) where a<120;

a b c b a
100 5 45 5 100
101 92 2 6 165
156 56 45 23 206
165 6 2 43 412
198 202 56 56 156
206 23 252 56 256
256 56 2 92 101
412 43 45 202 198

7

Q: count(*) where a<120;

a b c b a
100 5 45 5 100 100 5 45
101 92 2 6 165 101 92 2
156 56 45 23 206
165 6 2 43 412
198 202 56 56 156
206 23 252 56 256 2
256 56 2 92 101
412 43 45 202 198

7

Q: count(*) where b>50;

a b c b a
100 5 45 5 100
101 92 2 6 165
156 56 45 23 206
165 6 2 43 412
198 202 56 56 156
206 23 252 56 256
256 56 2 92 101
412 43 45 202 198

8

Q: count(*) where b>50;

a b c b a
56 156
100 5 45 5 100
56 256
101 92 2 6 165
92 101
156 56 45 23 206
202 198
165 6 2 43 412
198 202 56 56 156
206 23 252 56 256
4
256 56 2 92 101
412 43 45 202 198

8

Q: sum(c) where b>50;

a b c b a
100 5 45 5 100
101 92 2 6 165
156 56 45 23 206
165 6 2 43 412
198 202 56 56 156
206 23 252 56 256
256 56 2 92 101
412 43 45 202 198

9

56 156
56 256

Fast
a b c b a 92 101
100 5 45 5 100 202 198
101 92 2 6 165
156 56 45 23 206 156 56 45
165 6 2 43 412 256 56 2

Slow
198 202 56 56 156 101 92 2
206 23 252 56 256 198 202 56
256 56 2 92 101
412 43 45 202 198
105

9

What are indexes good for?
Indexes make queries go fast
• Each index will speed up some subset of queries

Design indexes with queries in mind
• Pick most important queries and set up indexes for those
• Consider the cost of maintaining the indexes

10

Goals of Talk
3 simple rules for designing good indexes

Avoid details of any data structure
• B-trees and Fractal Trees are interesting and fun for the
algorithmically minded computer scientist, but the 3 rules
will apply equally well to either data structure.
• All we need to care about is that range queries are fast
(per row) and that point queries are much slower (per
row).

11

The Rule to Rule Them All
There is no absolute rule
• Indexing is like a math problem
• Rules help, but each scenario is its own problem, which
requires problem-solving analysis

That said, rules help a lot

12

Three Basic Rules
1. Retrieve less data
• Less bandwidth, less processing, ...

2. Avoid point queries
• Not all data access cost is the same
• Sequential access is MUCH faster than random access

3. Avoid Sorting
• GROUP BY and ORDER BY queries do post-retrieval
work
• Indexing can help get rid of this work

13

Rule 1: Example of slow query
Example TABLE (1B rows, no indexes):
• create table foo (a int, b int, c int);

Query (1000 rows match):
• select sum(c) from foo where b=10 and a<150;

Query Plan:
• Rows where b=10 and a<150 can be anywhere in the table
• Without an index, entire table is scanned
Slow execution:
• Scan 1B rows just to count 1000 rows

Rule 1: How to add an index
What should we do?
• Reduce the data retrieved
• Analyze much less than 1B rows
How (for a simple select)?
• Design index by focusing on the WHERE clause
‣ This deﬁnes what rows the query is interested in
‣ Other rows are not important for this query

16

Rule 1: How to add an index
What should we do?
• Reduce the data retrieved
• Analyze much less than 1B rows
How (for a simple select)?
• Design index by focusing on the WHERE clause
‣ This deﬁnes what rows the query is interested in
‣ Other rows are not important for this query

select sum(c) from foo where b=10 and a<150;

16

Rule 1: Which index?
Option 1: key(a)
Option 2: key(b)
Which is better? Depends on selectivity:
• If there are fewer rows where a<150, then key(a) is better
• If there are fewer rows where b=10, then key(b) is better

Option 3: key(a) AND key(b), then MERGE
• We’ll come to this later

17

Rule 1: Picking the best key
Neither key(a) nor key(b) is optimal
Suppose:
• 200,000 rows exist where a<150
• 100,000 rows exist where b=10
• 1000 rows exist where b=10 and a<150
Then either index retrieves too much data

For better performance, indexes should try to
optimize over as many pieces of the where clause
as possible
• We need a composite index

18

Composite indexes reduce data retrieved
Where clause: b=5 and a<150
• Option 1: key(a,b)
• Option 2: key(b,a)
Which one is better?
• key(b,a)!
KEY RULE:
• When making a composite index, place equality checking
columns ﬁrst. Condition on b is equality, but not on a.

19

Q: where b=5 and a>150;

a b c b,a a
100 5 45 5,100 100
101 6 2 5,156 156
156 5 45 5,206 206
165 6 2 5,256 256
198 6 56 6,101 101
206 5 252 6,165 165
256 5 2 6,198 198
412 6 45 6,412 412

20

Composite Indexes: No equality clause
What if where clause is:
• where a>100 and a<200 and b>100;
Which is better?
• key(a), key(b), key(a,b), key(b,a)?

KEY RULE:
• As soon as a column on a composite index is NOT used for
equality, the rest of the composite index no longer reduces
data retrieved.
‣ key(a,b) is no better* than key(a)
‣ key(b,a) is no better* than key(b)

21

Composite Indexes: No equality clause
What if where clause is:
• where a>100 and a<200 and b>100;
Which is better?
• key(a), key(b), key(a,b), key(b,a)?

KEY RULE:
• As soon as a column on a composite index is NOT used for
equality, the rest of the composite index no longer reduces
data retrieved.
‣ key(a,b) is no better* than key(a)
‣ key(b,a) is no better* than key(b)
* Are there corner cases where it helps? Yes, but rare.

21

Q: where b>=5 and a>150;

a b c b,a a
100 5 45 5,100 100
101 6 2 5,156 156
156 5 45 5,206 206
165 6 2 5,256 256
198 6 56 6,101 101
206 5 252 6,165 165
256 5 2 6,198 198
412 6 45 6,412 412

22

Q: where b>=5 and a>150;

a b c b,a a
100 5 45 5,100 100
101 6 2 5,156 156 5,156 156
156 5 45 5,206 206 5,206 206
165 6 2 5,256 256 5,256 256
198 6 56 6,101 101 6,101 101
206 5 252 6,165 165 6,165 165
256 5 2 6,198 198 6,198 198
412 6 45 6,412 412 6,412 412

22

Composite Indexes: Another example
WHERE clause: b=5 and c=100
• key(b,a,c) is as good as key(b), because a is not used
in clause, so having c in index doesn’t help. key(b,c,a)
would be much better.

a b c b,a,c a
100 5 100 5,100,100 100
101 6 200 5,156,200 156
156 5 200 5,206,200 206
165 6 100 5,256,100 256
198 6 100 6,101,200 101
206 5 200 6,165,100 165
256 5 100 6,198,100 6,198
412 6 100 6,412,100 412
23

Composite Indexes: Another example
WHERE clause: b=5 and c=100
• key(b,a,c) is as good as key(b), because a is not used
in clause, so having c in index doesn’t help. key(b,c,a)
would be much better.

a b c b,a,c a
100 5 100 5,100,100 100 5,100,100 100
101 6 200 5,156,200 156 5,156,200 156
156 5 200 5,206,200 206 5,206,200 206
165 6 100 5,256,100 256 5,256,100 256
198 6 100 6,101,200 101
206 5 200 6,165,100 165
256 5 100 6,198,100 6,198
412 6 100 6,412,100 412
23

Rule 1: Recap
Goal is to reduce rows retrieved
• Create composite indexes based on where clause
• Place equality-comparison columns at the beginning
• Make ﬁrst non-equality column in index as selective as
possible
• Once ﬁrst column in a composite index is not used for
equality, or not used in where clause, the rest of the
composite index does not help reduce rows retrieved
‣ Does that mean they aren’t helpful?
‣ They might be very helpful... on to Rule 2.

24

Rule 2: Avoid point queries
Table:
• create table foo (a int, b int, c int,
primary key(a), key(b));
Query:
• select sum(c) from foo where b>50;
Query plan: use key(b)
• retrieval cost of each row is high because random point
queries are done

26


a b c b a
100 5 45 5 100
101 92 2 6 165
156 56 45 23 206
165 6 2 43 412
198 202 56 56 156
206 23 252 56 256
256 56 2 92 101
412 43 45 202 198

27

56 156
56 256
a b c b a 92 101
100 5 45 5 100 202 198
101 92 2 6 165
156 56 45 23 206 156 56 45
165 6 2 43 412 256 56 2
198 202 56 56 156 101 92 2
206 23 252 56 256 198 202 56
256 56 2 92 101
412 43 45 202 198
105

27

Rule 2: Avoid Point Queries
Table:
Query:
Query plan: scan primary table
• retrieval cost of each row is CHEAP!
• But you retrieve too many rows

28


a b c b a
100 5 45 5 100
101 92 2 6 165
156 56 45 23 206
165 6 2 43 412
198 202 56 56 156
206 23 252 56 256
256 56 2 92 101
412 43 45 202 198

29


100 5 45
a b c b a
101 92 2
100 5 45 5 100
156 56 45
101 92 2 6 165
165 6 2
156 56 45 23 206
198 202 56
165 6 2 43 412
206 23 252
198 202 56 56 156
256 56 2
206 23 252 56 256
412 43 45
256 56 2 92 101
412 43 45 202 198
105

29

Rule 2: Avoid Point Queries
Table:
Query:
What if we add another index?
• What about key(b,c)?
• Since we index on b, we retrieve only the rows we need.
• Since the index has information about c, we don’t need to
go to the main table. No point queries!

30


a b c b,c a
100 5 45 5,45 100
101 92 2 6,2 165
156 56 45 23,252 206
165 6 2 43,45 412
198 202 56 56,2 256
206 23 252 56,45 156
256 56 2 92,2 101
412 43 45 202,56 198

31


a b c b,c a
100 5 45 5,45 100
101 92 2 6,2 165 56,2 256
156 56 45 23,252 206 56,45 156
165 6 2 43,45 412 92,2 101
198 202 56 56,2 256 202,56 198
206 23 252 56,45 156
256 56 2 92,2 101
412 43 45 202,56 198 105

31

Covering Index
An index covers a query if the index has enough
information to answer the query.
Examples:
Q: select sum(c) from foo where b<100;
Q: select sum(d) from foo where b<100;
Indexes:
• key(b,c) -- covering index for ﬁrst query
• key(b,d) -- covering index for second query
• key(b,c,d) -- covering index for both

32

How to build a covering index
Add every ﬁeld from the select
• Not just where clause

Q: select c,d from foo where a=10 and
b=100;
Mistake: add index(a,b);
• This doesn’t cover the query. You still need point queries
to retrieve c and d values.

Correct: add index(a,b,c,d);
• Includes all referenced ﬁelds
• Place a and b at beginning by Rule 1

33

What if Primary Key matches where?
Q: select sum(c) from foo where b>100
and b<200;
Schema: create table foo (a int, b
int, c int, ai int auto_increment,
primary key (b,ai));
• Query does a range query on primary dictionary
• Only one dictionary is accessed, in sequential order
• This is fast

Primary key covers all queries
• If sort order matches where clause, problems solved

34

What’s a Clustering Index
What if primary key doesn’t match the where
clause?
• Ideally, you should be able to declare secondary indexes
that carry all ﬁelds
• AFAIK, storage engines don’t let you do this
• With one exception... TokuDB
• TokuDB allows you to declare any index to be
CLUSTERING
• A CLUSTERING index covers all queries, just like the
primary key

35

Clustering Indexes in Action
Q: select sum(d) from foo where b>200;
Q: select c,e from foo where b=1000;
Indexes:
• key(b,c) covers ﬁrst query
• key(b,d) covers second query
• key(b,c,e) covers ﬁrst and third queries
• key(b,c,d,e) covers all three queries

Indexes require a lot of analysis of queries

36

Clustering Indexes in Action
Q: select sum(d) from foo where b>200;
Q: select c,e from foo where b=1000;
What covers all queries?
• clustering key(b)

Clustering keys let you focus on the where
clause
• They eliminate point queries and make queries fast

37

More on clustering: Index Merge
We had example:
• create table foo(a int, b int, c int);
• select sum(c) from foo where b=10 and
a<150;

Suppose
• 200,000 rows have a<150
• 100,000 rows have b=10
• 1000 rows have b=10 and a<150

What if we use key(a) and key(b) and merge
results?

38

Query Plans
Merge plan:
• Scan 200,000 rows in key(a) where a<150
• Scan 100,000 rows in key(b) where b=10
• Merge the results and find 1000 row identifiers that
match query
• Do point queries with 1000 row identifiers to retrieve c

Better than no indexes
• Reduces number of rows scanned compared to no index
• Reduces number of point queries compared to not
merging

39

Does Clustering Help Merging?
Suppose key(a) is clustering
Query plan:
• Scan key(a) for 200,000 rows where a<150
• Scan resulting rows for ones where b=10
• Retrieve c values from 1000 remaining rows

Once again, no point queries
What’s even better?
• clustering key(b,a)!

40

Rule 2 Recap
Avoid Point Queries
Make sure index covers query
• By mentioning all ﬁelds in the query, not just those in the
where clause

Use clustering indexes
• Clustering indexes cover all queries
• Allows user to focus on where clause
• Speeds up more (and unforeseen) queries -- simpliﬁes
database design

41

Rule 3: Avoid Sorting
Simple selects require no post-processing
• select * from foo where b=100;

Just get the data and return to user
More complex queries do post-processing
• GROUP BY and ORDER BY sort the data

Index selection can avoid this sorting step

43

Avoid Sorting
Q1: select count(c) from foo;
Q2: select count(c) from foo group by
b, order by b;
Q1 plan:
• While doing a table scan, count rows with c

Q2 plan
• Scan table and write data to a temporary ﬁle
• Sort tmp ﬁle data by b
• Rescan sorted data, counting rows with c, for each b

44

Avoid Sorting
Q2: select count(c) from foo group by
b, order by b;
Q2: what if we use key(b,c)?
• By adding all needed fields, we cover query. FAST!
• By sorting first by b, we avoid sort. FAST!

Take home:
• Sort index on group by or order by fields to avoid
sorting

45

Summing Up
Use indexes to pre-sort for order by and
group by queries

46

Putting it all together
Sample Queries

Example
select count(*) from foo where c=5,
group by b;

48

Example
select count(*) from foo where c=5,
group by b;
key(c,b):
• First have c to limit rows retrieved (R1)
• Then have remaining rows sorted by b to avoid sort (R3)
• Remaining rows will be sorted by b because of equality
test on c

48

Example
select sum(d) from foo where c=100,
group by b;

49

Example
select sum(d) from foo where c=100,
group by b;
key(c,b,d):
• First have c to limit rows retrieved (R1)
• Then have remaining rows sorted by b to avoid sort (R3)
• Make sure index covers query, to avoid point queries (R2)

49

Example
Sometimes, there is no clear answer
• Best index is data dependent

Q: select count(*) from foo where
c<100, group by b;
Indexes:
• key(c,b)
• key(b,c)

50

Example
c<100, group by b;
Query plan for key(c,b):
• Will ﬁlter rows where c<100
• Still need to sort by b
‣ Rows retrieved will not be sorted by b
‣ where clause does not do an equality check on c, so b values are scattered in clumps for
different c values

51

Example
c<100, group by b;
Query plan for key(b,c):
• Sorted by b, so R3 is covered
• Rows where c>=100 are also processed, so not taking
advantage of R1

52

Example
Which is better?
• Answer depends on the data
• If there are many rows where c>=100, saving time by not
retrieving these useless rows helps. Use key(c,b).
• If there aren’t so many rows where c>=100, the time to
execute the query is dominated by sorting. Use
key(b,c).

The point is, in general, rules of thumb help, but
often they help us think about queries and
indexes, rather than giving a recipe.

53

Why not just load up
with indexes?
Need to keep up with insertion load
More indexes = smaller max load

Indexing cost
Space
• Issue
‣ Each index adds storage requirements
• Options
‣ Use compression (i.e., aggressive compression of 5-15x always on for
TokuDB)

Performance
• Issue
‣ B-trees while fast for certain indexing tasks (in memory, sequential keys),
are over 20x slower for other types of indexing
• Options
‣ Fractal Tree indexes (TokuDB’s data structure) is fast at all kinds of
indexing (i.e., random keys, large tables, wide keys, etc...)
‣ No need to worry about what type of index you are creating.
‣ Fractal trees enable customers to index early, index often

55

Last Caveat
Range Query Performance
• Issue
‣ Rule #2 (range query performance over point query performance)
depends on range queries being fast
‣ However B-trees can get fragmented
‣ from deletions, from random insertions, ...
‣ Fragmented B-trees get slow for range queries
• Options
‣ For B-trees, optimize tables, dump and reload, (ie, time consuming and
ofﬂine maintenance) ...
‣ For Fractal Tree indexing (TokuDB), not an issue
‣ Fractal trees don’t fragment

56

Thanks!
For more information...
• Please contact me at zardosht@tokutek.com for any
thoughts or feedback
• I will be presenting on indexing in April at both
Collaborate 11 in FL (10:30 am on 4/11) and the O’Reilly
Conference in CA (11:55 am on 4/12)
• Please visit Tokutek.com for a copy of this presentation
(goo.gl/S2LBe) to learn more about the power of
indexing, read about Fractal Tree indexes, or to download
a free eval copy of TokuDB

57

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