explain() in Mongodb: differences between “nscanned” and “nscannedObjects”
I cannot get the exact difference between "nscanned" and "nscannedObjects" in the Mongodb's explain query output.
On MongoDB Explain documentation I can read:
nscanned Number of items (documents or index entries) examined. Items
might be objects or index keys. If a "covered index" is involved,
nscanned may be higher than nscannedObjects.
nscannedObjects Number of documents scanned.
What's the different between these two fields?
And more specific what does exactly mean when I have a query, which uses a BtreeCursor (an index), and these two fields have two different values, for example:
{
"cursor" : "BtreeCursor a_1_b_1",
"isMultiKey" : false,
"n" : 5,
"nscannedObjects" : 5,
"nscanned" : 9,
(...)
}
I know what a "covered index" is.
I would like to understand exactly what the query did in the example above.
Did it pass through ("scanned") 9 elements (nscanned = 9), where all of them are index entries and read ("examined") the value of only 5 of them (nscannedObjects = 5) to produce the result set?
mongodb
edited May 23 '17 at 12:17
Community♦
11
asked Dec 17 '12 at 7:40
Luigi Massa GalleranoLuigi Massa Gallerano
1,58011524
add a comment |
I cannot get the exact difference between "nscanned" and "nscannedObjects" in the Mongodb's explain query output.
On MongoDB Explain documentation I can read:
nscanned Number of items (documents or index entries) examined. Items
might be objects or index keys. If a "covered index" is involved,
nscanned may be higher than nscannedObjects.
nscannedObjects Number of documents scanned.
What's the different between these two fields?
And more specific what does exactly mean when I have a query, which uses a BtreeCursor (an index), and these two fields have two different values, for example:
{
"cursor" : "BtreeCursor a_1_b_1",
"isMultiKey" : false,
"n" : 5,
"nscannedObjects" : 5,
"nscanned" : 9,
(...)
}
I know what a "covered index" is.
I would like to understand exactly what the query did in the example above.
Did it pass through ("scanned") 9 elements (nscanned = 9), where all of them are index entries and read ("examined") the value of only 5 of them (nscannedObjects = 5) to produce the result set?
mongodb
edited May 23 '17 at 12:17
Community♦
11
asked Dec 17 '12 at 7:40
Luigi Massa GalleranoLuigi Massa Gallerano
1,58011524
add a comment |
I cannot get the exact difference between "nscanned" and "nscannedObjects" in the Mongodb's explain query output.
On MongoDB Explain documentation I can read:
nscanned Number of items (documents or index entries) examined. Items
might be objects or index keys. If a "covered index" is involved,
nscanned may be higher than nscannedObjects.
nscannedObjects Number of documents scanned.
What's the different between these two fields?
And more specific what does exactly mean when I have a query, which uses a BtreeCursor (an index), and these two fields have two different values, for example:
{
"cursor" : "BtreeCursor a_1_b_1",
"isMultiKey" : false,
"n" : 5,
"nscannedObjects" : 5,
"nscanned" : 9,
(...)
}
I know what a "covered index" is.
I would like to understand exactly what the query did in the example above.
Did it pass through ("scanned") 9 elements (nscanned = 9), where all of them are index entries and read ("examined") the value of only 5 of them (nscannedObjects = 5) to produce the result set?
mongodb
edited May 23 '17 at 12:17
Community♦
11
asked Dec 17 '12 at 7:40
Luigi Massa GalleranoLuigi Massa Gallerano
1,58011524
I cannot get the exact difference between "nscanned" and "nscannedObjects" in the Mongodb's explain query output.
On MongoDB Explain documentation I can read:
nscanned Number of items (documents or index entries) examined. Items
might be objects or index keys. If a "covered index" is involved,
nscanned may be higher than nscannedObjects.
nscannedObjects Number of documents scanned.
What's the different between these two fields?
And more specific what does exactly mean when I have a query, which uses a BtreeCursor (an index), and these two fields have two different values, for example:
{
"cursor" : "BtreeCursor a_1_b_1",
"isMultiKey" : false,
"n" : 5,
"nscannedObjects" : 5,
"nscanned" : 9,
(...)
}
I know what a "covered index" is.
I would like to understand exactly what the query did in the example above.
Did it pass through ("scanned") 9 elements (nscanned = 9), where all of them are index entries and read ("examined") the value of only 5 of them (nscannedObjects = 5) to produce the result set?
mongodb
mongodb
edited May 23 '17 at 12:17
Community♦
11
asked Dec 17 '12 at 7:40
Luigi Massa GalleranoLuigi Massa Gallerano
1,58011524
edited May 23 '17 at 12:17
Community♦
11
asked Dec 17 '12 at 7:40
Luigi Massa GalleranoLuigi Massa Gallerano
1,58011524
edited May 23 '17 at 12:17
Community♦
11
edited May 23 '17 at 12:17
Community♦
11
edited May 23 '17 at 12:17
Community♦
11
11
asked Dec 17 '12 at 7:40
Luigi Massa GalleranoLuigi Massa Gallerano
1,58011524
asked Dec 17 '12 at 7:40
Luigi Massa GalleranoLuigi Massa Gallerano
1,58011524
asked Dec 17 '12 at 7:40
Luigi Massa GalleranoLuigi Massa Gallerano
1,58011524
1,58011524
add a comment |
add a comment |
2 Answers
2
active
oldest
votes
This means that :
The query returned 5 documents - n
scanned 9 documents from the index - nscanned
and then read 5 full documents from the collection - nscannedObjects
Similar example is given at :
http://docs.mongodb.org/manual/reference/method/cursor.explain/#cursor.explain
answered Dec 17 '12 at 8:04
Ajay GeorgeAjay George
9,63713244
add a comment |
if "cursor" is Index ==> nscanned = no. of index scanned
else if "cursor" is FullTableScan ==> nscanned = no. of document scanned
nscannedObjects ==> No. of document scanned
when querying, try to minimise all count, i.e. nscanned and nscannedObjects both are minimum means your query should run faster!
answered Dec 17 '12 at 14:58
Raxit ShethRaxit Sheth
89011017
add a comment |
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
This means that :
The query returned 5 documents - n
scanned 9 documents from the index - nscanned
and then read 5 full documents from the collection - nscannedObjects
Similar example is given at :
http://docs.mongodb.org/manual/reference/method/cursor.explain/#cursor.explain
answered Dec 17 '12 at 8:04
Ajay GeorgeAjay George
9,63713244
add a comment |
This means that :
The query returned 5 documents - n
scanned 9 documents from the index - nscanned
and then read 5 full documents from the collection - nscannedObjects
Similar example is given at :
http://docs.mongodb.org/manual/reference/method/cursor.explain/#cursor.explain
answered Dec 17 '12 at 8:04
Ajay GeorgeAjay George
9,63713244
add a comment |
This means that :
The query returned 5 documents - n
scanned 9 documents from the index - nscanned
and then read 5 full documents from the collection - nscannedObjects
Similar example is given at :
http://docs.mongodb.org/manual/reference/method/cursor.explain/#cursor.explain
answered Dec 17 '12 at 8:04
Ajay GeorgeAjay George
9,63713244
This means that :
The query returned 5 documents - n
scanned 9 documents from the index - nscanned
and then read 5 full documents from the collection - nscannedObjects
Similar example is given at :
http://docs.mongodb.org/manual/reference/method/cursor.explain/#cursor.explain
answered Dec 17 '12 at 8:04
Ajay GeorgeAjay George
9,63713244
edited Oct 7 '14 at 16:01
answered Dec 17 '12 at 8:04
Ajay GeorgeAjay George
9,63713244
answered Dec 17 '12 at 8:04
Ajay GeorgeAjay George
9,63713244
answered Dec 17 '12 at 8:04
Ajay GeorgeAjay George
9,63713244
9,63713244
add a comment |
add a comment |
if "cursor" is Index ==> nscanned = no. of index scanned
else if "cursor" is FullTableScan ==> nscanned = no. of document scanned
nscannedObjects ==> No. of document scanned
when querying, try to minimise all count, i.e. nscanned and nscannedObjects both are minimum means your query should run faster!
answered Dec 17 '12 at 14:58
Raxit ShethRaxit Sheth
89011017
add a comment |
if "cursor" is Index ==> nscanned = no. of index scanned
else if "cursor" is FullTableScan ==> nscanned = no. of document scanned
nscannedObjects ==> No. of document scanned
when querying, try to minimise all count, i.e. nscanned and nscannedObjects both are minimum means your query should run faster!
answered Dec 17 '12 at 14:58
Raxit ShethRaxit Sheth
89011017
add a comment |
if "cursor" is Index ==> nscanned = no. of index scanned
else if "cursor" is FullTableScan ==> nscanned = no. of document scanned
nscannedObjects ==> No. of document scanned
when querying, try to minimise all count, i.e. nscanned and nscannedObjects both are minimum means your query should run faster!
answered Dec 17 '12 at 14:58
Raxit ShethRaxit Sheth
89011017
if "cursor" is Index ==> nscanned = no. of index scanned
else if "cursor" is FullTableScan ==> nscanned = no. of document scanned
nscannedObjects ==> No. of document scanned
when querying, try to minimise all count, i.e. nscanned and nscannedObjects both are minimum means your query should run faster!
answered Dec 17 '12 at 14:58
Raxit ShethRaxit Sheth
89011017
answered Dec 17 '12 at 14:58
Raxit ShethRaxit Sheth
89011017
answered Dec 17 '12 at 14:58
Raxit ShethRaxit Sheth
89011017
answered Dec 17 '12 at 14:58
Raxit ShethRaxit Sheth
89011017
89011017
add a comment |
add a comment |
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