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mention howto use indices for joins (#3229)

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Wilfried Goesgens 2017-09-13 17:45:45 +02:00 committed by Frank Celler
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Documentation/Books/AQL/Examples/Join.md
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@ -220,6 +220,16 @@ So, for each user we pick the list of their friends and count them. The ones whe
count equals zero are the lonely people. Using *RETURN 1* in the subquery
saves even more precious CPU cycles and gives the optimizer more alternatives.
### Index usage
Especially on joins you should [make sure indices can be used to speed up your query.](../../ExecutionAndPerformance/ExplainingQueries.md)
Please note that sparse indices don't qualify for joins:
In joins you typically would also want to join documents not containing the property
you join with. However sparse indices don't contain references to documents that
don't contain the indexed attributes - thus they would be missing from the join operation.
For that reason you should provide non-sparse indices.
### Pitfalls
Since we're free of schemata, there is by default no way to tell the format of the