[SERVER-12219] Use of $isolated inhibits query by id runner selection Created: 31/Dec/13 Updated: 11/Jul/16 Resolved: 10/Jan/14 |
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| Status: | Closed |
| Project: | Core Server |
| Component/s: | Querying |
| Affects Version/s: | None |
| Fix Version/s: | 2.5.5 |
| Type: | Bug | Priority: | Major - P3 |
| Reporter: | Andrew Morrow (Inactive) | Assignee: | David Storch |
| Resolution: | Done | Votes: | 0 |
| Labels: | Performance | ||
| Remaining Estimate: | Not Specified | ||
| Time Spent: | Not Specified | ||
| Original Estimate: | Not Specified | ||
| Backwards Compatibility: | Fully Compatible |
| Operating System: | ALL |
| Steps To Reproduce: | Run an _id query driven workload under 'perf top', and notice the IDHackRunner in the output. Re-run with $isolated, and note that the IDHackRunner goes away, and the QueryPlanner::plan function appears. Also, if you measure throughput, it is noticeably slower. |
| Participants: |
| Description |
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Normally, a query like { _id : 1 }will be routed to the "id hack" query runner. However, if the query is marked isolated with { _id : 1, $isolated : 1 }, then the query execution is routed through QueryPlanner::plan, rather than redirected to the id hack runner, which greatly reduces performance. |
| Comments |
| Comment by Githook User [ 10/Jan/14 ] |
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Author: {u'username': u'dstorch', u'name': u'David Storch', u'email': u'david.storch@10gen.com'}Message: Signed-off-by: Matt Kangas <matt.kangas@mongodb.com> |
| Comment by Andrew Morrow (Inactive) [ 09/Jan/14 ] |
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It is syntactically valid, so while it may be redundant for a query by _id, it should be harmless to specify it, but it currently is harmful. The alternative would be to forbid it, but on what basis? |
| Comment by Asya Kamsky [ 09/Jan/14 ] |
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$isolated doesn't make sense for a query by _id. the point of $isolated is to not yield between multiple document updates (for example). How can $isolated make sense with _id which by definition will only match one document? |