sequence.reduce(function) → value r.reduce(sequence, function) → value
Produce a single value from a sequence through repeated application of a reduction function.
The reduction function may be called on:
The reduction function can be called on the results of two previous reductions because the reduce command is distributed and parallelized across shards and CPU cores. A common mistaken when using the reduce command is to suppose that the reduction is executed from left to right. Read the map-reduce in RethinkDB article to see an example.
If the sequence is empty, the server will produce a ReqlRuntimeError that can be caught with default_. If the sequence has only one element, the first element will be returned.
Example: Return the number of documents in the table posts.
r.table("posts").map(doc -> 1).reduce(
(left, right) -> left.add(right)
).default_(0).run(conn);
A shorter way to execute this query is to use count.
Example: Suppose that each post has a field comments that is an array of comments.
Return the number of comments for all posts.
r.table("posts").map(doc -> doc.g("comments").count()).reduce(
(left, right) -> left.add(right)
).default_(0).run(conn);
Example: Suppose that each post has a field comments that is an array of comments.
Return the maximum number comments per post.
r.table("posts").map(doc -> doc.g("comments").count()).reduce(
(left, right) -> r.branch(left.gt(right), left, right)
).default_(0).run(conn);
A shorter way to execute this query is to use max.
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https://rethinkdb.com/api/java/reduce/