Create Index for optimized answer retreival

[dedenbangkit]

In order to use projection queries for answers, we need to create the following index:

The suggested index for this query is:

<datastore-index kind="QuestionAnswerStore" ancestor="false" source="manual">
    <property name="questionID" direction="asc"/>
    <property name="surveyInstanceId" direction="asc"/>
    <property name="value" direction="asc"/>
</datastore-index>

See: akvo/akvo-flow-api#244

[stale]

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