1. Indexing
Mike
Dirolf
-­‐
@mdirolf
-­‐
10gen,
Inc.
http://www.mongodb.org

2. What’s Easy About
MongoDB Indexing?
It’s
almost
the
same
as
in
your
RDBMS

3. What’s Hard About
MongoDB Indexing?
It’s
almost
the
same
as
in
your
RDBMS

4. What is an Index?
Magic
scaling
sauce?

5. What is an Index?
A
data
structure
that
can
be
used
to
make
certain
queries
more
efficient.

6. Indexes Maintain Order
Index
on
{a:
1}
{a:
0,
b:
9}
{a:
2,
b:
0}
{a:
3,
b:
2}
{a:
3,
b:
7}
{a:
3,
b:
5}
{a:
7,
b:
1}
{a:
9,
b:
1}

7. Indexes Maintain Order
Index
on
{a:
1,
b:
-­‐1}
{a:
0,
b:
9}
{a:
2,
b:
0}
{a:
3,
b:
7}
{a:
3,
b:
5}
{a:
3,
b:
2}
{a:
7,
b:
1}
{a:
9,
b:
1}

8. B-tree Structure
Index
on
{a:
1}
[-∞, 5) [5, 10) [10, ∞)
[-∞, 5) buckets [5, 7) [7, 9) [9, 10) [10, ∞) buckets
{...}
{...}
{...}
{...}
{...}
{...}
{...}
{...}
{...}
{...}
{...}

9. Query for {a: 7}
Index
[-∞, 5) [5, 10) [10, ∞)
[-∞, 5) buckets [5, 7) [7, 9) [9, 10) [10, ∞) buckets
{...}
{...}
{...}
{...}
{...}
{...}
{...}
{...}
{...}
{...}
{...}
Scan

10. Creating Indexes
An
index
on
_id
is
automatic.
For
more,
ensureIndex:
db.posts.ensureIndex({“name”:
1})

11. Compound Indexes
db.posts.ensureIndex({name:
1,
date:
-­‐1})

12. Unique Indexes
db.posts.ensureIndex({title:
1},
{unique:
true})

13. Background Builds
db.posts.ensureIndex(...,
{background:
true})

14. Indexing Embedded
Documents
db.posts.ensureIndex({“comments.author”:
1})

15. Multikeys
{“tags”:
[“mongodb”,
“indexing”],
...}
db.posts.ensureIndex({“tags”:
1})

16. Geospatial
db.posts.ensureIndex({“location”:
“2d”})

17. Listing Indexes
db.posts.getIndexes()

18. Dropping an Index
db.posts.dropIndex({“tags”:
1})

19. When is an Index Used?
Index
on
{a:
1}
db.collection.find({a:
0})
db.collection.find({a:
{$in:
[0,
2]}})
db.collection.find({a:
{$gt:
5}})
db.collection.count({a:
0})
db.collection.find().sort({a:
-­‐1})
Partially:
db.collection.find({b:
0}).sort({a:
-­‐1})

20. When isn’t an Index Used?
Index
on
{a:
1,
b:
-­‐1}
db.collection.find({b:
0})
As
a
rule:
try
imagining
how
the
sorted
representation
could
help
the
server
with
your
query.

21. Picking an Index
find({x:
10,
y:
“foo”})
scan
terminate
index
on
x
index
on
y remember

22. When are Indexes
Needed?
Frequently
used
queries
Low
response
time

23. Indexes Take Up
Space
db.collection.totalIndexSize()

24. Indexes Slow Down
Writes

25. Explain
db.collection.find(query).explain();
{
"cursor"
:
"BasicCursor",
"indexBounds"
:
[
],
"nscanned"
:
57594,
"nscannedObjects"
:
57594,
"n"
:
3
,
"millis"
:
108
}

26. Explain
{
"cursor"
:
"BtreeCursor
x_1",
"indexBounds"
:
[
],
"nscanned"
:
123,
"nscannedObjects"
:
123,
"n"
:
10
,
"millis"
:
4
}

27. www.mongodb.org