parallelCollectionScan¶

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Example
Output

parallelCollectionScan¶

2.6 新版功能.

Allows applications to use multiple parallel cursors when reading all the documents from a collection, thereby increasing throughput. The parallelCollectionScan command returns a document that contains an array of cursor information.

Each cursor provides access to the return of a partial set of documents from a collection. Iterating each cursor returns every document in the collection. Cursors do not contain the results of the database command. The result of the database command identifies the cursors, but does not contain or constitute the cursors.

The server may return fewer cursors than requested.

The command has the following syntax:

{
  parallelCollectionScan: "<collection>",
  numCursors: <integer>
}

The parallelCollectionScan command takes the following fields:

Field Type Description

parallelCollectionScan string The name of the collection.

numCursors integer The maximum number of cursors to return. Must be between 1 and 10000, inclusive.

readConcern

document

Optional. Specifies the read concern. The option has the following syntax:

readConcern: { level: <value> }

Possible read concern values are:

"local". This is the default read concern level.
"majority". Available for replica sets that use WiredTiger storage engine.
"linearizable". Available for read operations on the primary only.

For more formation on the read concern levels, see Read Concern Levels.

parallelCollectionScan is only available for mongod, and it cannot operate on a sharded cluster.

Example¶

Override Default Read Concern¶

To override the default read concern level of "local", use the readConcern option.

The following operation on a replica set specifies a Read Concern of "majority" to read the most recent copy of the data confirmed as having been written to a majority of the nodes.

注解

To use read concern level of "majority",
- you must start the mongod instances with the --enableMajorityReadConcern command line option (or the replication.enableMajorityReadConcern set to true if using a configuration file).
- replica sets must use WiredTiger storage engine and election protocol version 1.
Regardless of the read concern level, the most recent data on a node may not reflect the most recent version of the data in the system.

db.runCommand(
   {
      parallelCollectionScan: "restaurants",
      numCursors: 5,
      readConcern: { level: "majority" }
   }
)

To ensure that a single thread can read its own writes, use "majority" read concern and "majority" write concern against the primary of the replica set.

Output¶

The parallelCollectionScan command returns a document containing the array of cursor information:

{
    "cursors" : [
        {
            "cursor" : {
                "firstBatch" : [ ],
                "ns" : "<database name>.<collection name>",
                "id" : NumberLong("155949257847")
            },
            "ok" : true
        }
    ],
    "ok" : 1
}

parallelCollectionScan.cursors¶: An array with one or more cursors returned with the command.

parallelCollectionScan.cursors.cursor¶: For each cursor returned, a document with details about the cursor.

parallelCollectionScan.cursors.cursor.firstBatch¶: An empty first batch is useful for quickly returning a cursor or failure message without doing significant server-side work. See cursor batches.

parallelCollectionScan.cursors.cursor.ns¶: The namespace for each cursor.

parallelCollectionScan.cursors.cursor.id¶: The unique id for each cursor.

parallelCollectionScan.cursors.ok¶: The status of each cursor returned with the command.

parallelCollectionScan.ok¶: A value of 1 indicates the parallelCollectionScan command succeeded. A value of 0 indicates an error.

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