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https://issues.apache.org/jira/browse/TINKERPOP-2917?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17733205#comment-17733205
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Stephen Mallette commented on TINKERPOP-2917:
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[~martin.haeusler] did you get a chance to look at my comment above? does that
solve your problem?
> Please make specification of "valueMap()" step more index-friendly
> ------------------------------------------------------------------
>
> Key: TINKERPOP-2917
> URL: https://issues.apache.org/jira/browse/TINKERPOP-2917
> Project: TinkerPop
> Issue Type: Improvement
> Components: language
> Affects Versions: 3.6.2
> Reporter: Martin Häusler
> Priority: Major
>
> The {{valueMap(...)}} step states that it returns the property values found
> in the graph element. There's nothing inherently wrong with this definition,
> however it doesn't lend itself well to index-supported execution. For example:
> {code:java}
> traversal().V().has("type", "person").valueMap("firstName", "lastName")
> {code}
> This traversal may be executed on very large graphs entirely without loading
> a single element from disk, provided that {{{}type{}}}, {{firstName}} and
> {{lastName}} are indexed. That would be the ideal case, as it is roughly
> equivalent to a (commonly used) SQL query like this:
> {code:sql}
> SELECT firstName, lastName
> FROM person
> {code}
> The problems come in when the {{valueMap}} step should be answered from the
> secondary index:
> 1. Most secondary indices cannot distinguish between a list of "John", a set
> of "John", or the raw string value "John" (each entry of a multi-valued
> property is indexed separately).
> 2. Most secondary indices cannot distinguish between {{[ "John", "John" ]}}
> and {{["John"]}} (same values are written to the index only once per vertex).
> Issue 1 could be overcome if we had a schema on the graph, but for a
> schemaless graph there's no way to tell if the index result should be a list,
> a set or a raw string. Issue 2 isn't even solved if we had a schema (we would
> know that the property is a List, but we would have no way to tell how often
> a given value appears in the list).
> So, how can we answer {{valueMap()}} with index support? In ChronoGraph, I've
> introduced a setting {{{}USE_SECONDARY_INDEX_FOR_VALUE_MAP_STEP{}}}. If this
> setting is active, we deviate slightly from the gremlin {{valueMap()}}
> definition:
> - Each value will always be reported as a {{{}Set{}}}, even if the original
> vertex property was a single value or a List (i.e. only distinct values will
> be reported, in arbitrary order).
> - If the value was {{null}} or the property didn't exist on the vertex at
> all, an empty set is reported.
> This isn't harmful (as far as I can tell), but it is a difference to the
> standard behavior. In exchange, we get the possibility to scan our secondary
> indices to answer {{valueMap()}} steps, which is a great advantage in
> performance.
>
> I would very much like to see an adaptation of the standard (for example, the
> way I've described above) to make it more friendly towards index usage.
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