How does skipping work in a sharded collection?

[Tomas vd W]

I have a question regarding skipping and sharding. It works perfectly,
but I don't understand how.

I have a simple test setup with 3 shards and a collection sharded by
_id.

When I do a sort/skip/limit operation:

db.mycoll.find().sort({_id: 1}).skip(100).limit(10).explain()

I would expect that each shard would read and send 110 records to
mongos, and that mongos would perform the skip while zipping these 330
records together. But explain() shows that each shard reads and sends
only 10 records to mongos. How do the individual shards know how much
to skip?

Furthermore, if they know how much to skip, why do they all need to
send 10 records? What are the 20 redundent records for?

Could someone explain this?

Thanks,
Tomas

[Scott Hernandez]

Skip is applied before limit so each shard does that first. Therefore
they all skip the correct and simply return 10. It could be that only
one of the shards returns all results, or that the are interleaved at
mongos depending on the sort.

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[songhe yang]

On Jan 31, 11:17 pm, Scott Hernandez <scotthernan...@gmail.com> wrote:
> Skip is applied before limit so each shard does that first. Therefore
> they all skip the correct and simply return 10.

You means that the skip operation was executed on mongod ?

But if each mongod skip 100 documents, the result will not correct.

So i think the skip was executed on mongos.


Hopeful more detail explain about it, 3x

> It could be that only
> one of the shards returns all results, or that the are interleaved at
> mongos depending on the sort.
>

[Scott Hernandez]

No, it is run on each shard and then aggregated. It is possible that
the result came from just one shard depending on the sort and shard
key.

[songhe yang]

How each shard know the skip number ? I think that each shard should
not just skip 100.