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Re: [geomesa-users] Executing a query - process
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Hi Marcel,
Executing a query is basically 3 steps:
1. Choose the best index to scan given the query
2. Set up the scan ranges, iterators, etc for the given index
3. Transform the results coming back from accumulo into SimpleFeatures
- We use a cost-based strategy for picking an index to scan. Right now
it is fairly static, but we allow for user hints to indicate a preferred
strategy. The code for calculating costs is here:
https://github.com/locationtech/geomesa/blob/master/geomesa-accumulo/geomesa-accumulo-datastore/src/main/scala/org/locationtech/geomesa/accumulo/index/QueryStrategyDecider.scala
- We use the feature ID out of the index table in order to 'join'
against the record table. You can see that code snippet here:
https://github.com/locationtech/geomesa/blob/master/geomesa-accumulo/geomesa-accumulo-datastore/src/main/scala/org/locationtech/geomesa/accumulo/index/AttributeIdxStrategy.scala#L153-L154
- When scanning the records table, we only care about the Value - that
is the serialized simple feature, including ID. The row of the record
table has the feature ID in it, which is how we set up a range to scan.
Hope that helps,
Emilio
On 10/21/2015 11:59 AM, Marcel wrote:
Hello all,
I´m struggling how the process of executing a query is done on a
theoretical level.
Could you explain how Geomesa executes this query (step by step)?
Following points would be very interesting:
- Is there an order for the tables? (maybe look first at attr_idx,
then st_idx and finally full table scan if needed...something like this).
- What is returned from the index table? (for mapping with records
table - maybe feature id?)
- What about the records table? (current structure of rowid -
including feature id?)
Best regards,
Marcel Jacob.
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