lksvenoy-r7 opened a new issue, #9082:
URL: https://github.com/apache/pinot/issues/9082
Currently, the star-tree index can not be used with multi-valued dimensional
columns. The problem with supporting multi-valued columns in the split order of
the star-tree index is that the behaviour for splitting on multiple values in
the split-order would be defined, as the behaviour is no longer additive.
This ticket is a suggestion to implement support for multi-valued
dimensional columns in the split order for the star-tree index, with a simply
constraint of only allowing one specific value to be used when splitting for
the index.
Suppose we have a table which has the following dimensions:
```
entityId: String
groupIds: List of strings
status: String
```
And a query looking like this
`SELECT COUNT(*) FROM table WHERE entityId = 'ID' AND groupIds CONTAINS
'group_1' AND status = 'ONLINE'`
If there are many rows here, this could be a very intense query to resolve.
If the star tree index could pre-aggregate on each value in groupId, we could
make this lookup much more performant. This is a problem that we are facing
today. A group can contain multiple entities, and an entity can be in multiple
groups. This many to many relationship means there is a dynamic constraint on
our queries.
Some queries will ignore group (that would be the star node), while some
need to access the aggregation on a group basis.
The problem with implementing support for this, is that it wouldn't work for
a query defined as this:
`SELECT COUNT(*) FROM table WHERE entityId = 'ID' AND groupIds IN
('group_1', 'group_2') AND status = 'ONLINE'`
This behaviour would be undefined, as there is no way to create one
aggregation per groupId if we also need to handle all the permutations of
possible values in the multi-valued column. This is why I propose that we
support multi-valued columns for the star-tree index with the constraint that
each value would have an aggregation associated with it, but not allowing it to
span a selection of multiple values.
For example..
With the following star-tree index:
```
"starTreeIndexConfigs": [
{
"dimensionsSplitOrder": [
"entityId",
"groupIds",
"status"
],
"skipStarNodeCreationForDimensions": [],
"functionColumnPairs": [
"COUNT__*"
],
"maxLeafRecords": 10000
}
],
```
This would be possible
```
entity_1 -> count(*) = 500
entity_1 -> group_1 -> count(*) = (200)
entity_1 -> group_1 -> ONLINE -> count(*) = 100
entity_1 -> group_1 -> OFFLINE -> count(*) = 100
entity_1 -> group_2 -> count(*) = (300)
entity_1 -> group_2 -> ONLINE -> count(*) = 150
entity_1 -> group_2 -> OFFLINE -> count(*) = 150
```
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