How Mongos routes queries

# example query
db.products.find({"name": "How To Mongo"})

All queries goes to mongos when querying a cluster.

1. determine list of shards that must recieve the query.

• if query includes shard key, the query can specificly target related shards. This is very efficient
• if query does NOT include shard key, mongos targets all shards in the cluster. this can be slow depending on # of shards in the cluster

2. mongos ALWAYS opens a cursor against 'each' of the targeted shards. Each cursor queries && returns data for the given shard(s).
3. mongos merges results to fulfill the query.
4. mongos returns the data

Mongos Cursor Specifics

• sort
  • pushes sort to each shard in the cluster
  • performs a merge-sort on results
• limit
  • pushes limit to each shard
  • reapplies limit after returning results
• skip
  • performs skipping on merged results - nothing goes down to sharding level
• limit + skip
  • when both are present they BOTH get passed to the shard(s)

Targeted vs ScatterGather

Each shard contains chunks of sharded data.
Inclusive lower bound.
Exclusive upper bound.
Targeted Queries are faster than Scatter+Gather.
Ranged queries on hashed shard keys require the scatter-gather.
Compound indexes -

• ex. sku, type & name fields
• EACH of the fields can be used to perform a targeted query when in the shard-key-listed order
  • ex. when listed as shard key value {sku:1, type:1, name:1}
    • querying for this will target the query:db.products.find({sku:234})
    • querying for this will target the query:db.products.find({sku:234, type: 'banana'})
    • querying for this will target the query:db.products.find({sku:234, type: 'banana', name: 'pinto'})
    • querying this will NOT target the query: db.products.find({type: 'banana', name: 'pinto'})
    • querying this will NOT target the query: db.products.find({type: 'banana'})
    • querying this will NOT target the query: db.products.find({name: 'pinto'})

SEE which fields have triggered targeted queries

db.products.find({sku: 100000749}).explain()

# will show that the scanning takes advantage of the sku
# including IXSCAN in the stage


db.products.find( {
"name" : "Gods And Heroes: Rome Rising - Windows [Digital Download]" }
).explain()

# will show COLLSCAN as a step, a slower step
# with multiple shards, this will scan all shards