Thomas Mueller created OAK-446:
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Summary: Query: implement an intersecting index
Key: OAK-446
URL: https://issues.apache.org/jira/browse/OAK-446
Project: Jackrabbit Oak
Issue Type: Bug
Reporter: Thomas Mueller
Assignee: Thomas Mueller
In order to run queries with multiple conditions efficiently, it is currently
required to create an index on all of those conditions. For example, the query:
{code}
where lastName = 'x' and firstName = 'y'
{code}
will only run efficiently (assuming there are many nodes with the same lastName
and many nodes with the same firstName) if there is an index on both lastName
_and_ firstName. If there are two indexes, one just lastName and the other on
just firstName, then one of those index is used, but not both.
The problem doesn't only apply to properties, it also applies to nodeType. So a
query of the form
{code}
select * from [acme:Page] where [x] = 'y'
{code}
will use either an index on the node type, or an index on 'x', but not both. It
seems such queries are quite important in JCR.
To speed up such queries, I suggest we implement a (virtual) 'intersecting
index' that internally merges the results from multiple (two or more) indexes.
To do that, the indexes need to have a common property, for example the path.
For example, the first index is on lastName and path, the second index on
firstName and path. The intersecting index would then first query the first
index with firstName = 'x', and then query the second index, with lastName =
'y' and path >= '...' (the value returned by the first index). This would go
back and forth until a row is found that satisfies both conditions (the
intersection could be empty of course).
To make this work, index implementations should support path lookup.
To speed up cost calculation for the intersecting index, it might be needed to
extend the QueryIndex interface to return the list of property restrictions an
index supports.
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