druid/docs/querying/limitspec.md

71 lines
3.1 KiB
Markdown
Raw Normal View History

---
id: limitspec
title: "Sorting and limiting (groupBy)"
---
<!--
~ Licensed to the Apache Software Foundation (ASF) under one
~ or more contributor license agreements. See the NOTICE file
~ distributed with this work for additional information
~ regarding copyright ownership. The ASF licenses this file
~ to you under the Apache License, Version 2.0 (the
~ "License"); you may not use this file except in compliance
~ with the License. You may obtain a copy of the License at
~
~ http://www.apache.org/licenses/LICENSE-2.0
~
~ Unless required by applicable law or agreed to in writing,
~ software distributed under the License is distributed on an
~ "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
~ KIND, either express or implied. See the License for the
~ specific language governing permissions and limitations
~ under the License.
-->
> Apache Druid supports two query languages: [Druid SQL](sql.md) and [native queries](querying.md).
> This document describes the native
> language. For information about sorting in SQL, refer to the [SQL documentation](sql-syntax.md#order-by).
The limitSpec field provides the functionality to sort and limit the set of results from a groupBy query. If you group by a single dimension and are ordering by a single metric, we highly recommend using [TopN Queries](../querying/topnquery.md) instead. The performance will be substantially better. Available options are:
### DefaultLimitSpec
The default limit spec takes a limit and the list of columns to do an orderBy operation over. The grammar is:
```json
{
"type" : "default",
"limit" : <optional integer>,
"offset" : <optional integer>,
"columns" : [<optional list of OrderByColumnSpec>],
}
```
The "limit" parameter is the maximum number of rows to return.
The "offset" parameter tells Druid to skip this many rows when returning results. If both "limit" and "offset" are
provided, then "offset" will be applied first, followed by "limit". For example, a spec with limit 100 and offset 10
will return 100 rows starting from row number 10. Internally, the query is executed by extending the limit by the offset
and then discarding a number of rows equal to the offset. This means that raising the offset will increase resource
usage by an amount similar to increasing the limit.
Together, "limit" and "offset" can be used to implement pagination. However, note that if the underlying datasource is
modified in between page fetches in ways that affect overall query results, then the different pages will not
necessarily align with each other.
#### OrderByColumnSpec
OrderByColumnSpecs indicate how to do order by operations. Each order-by condition can be a `jsonString` or a map of the following form:
```json
{
"dimension" : "<Any dimension or metric name>",
"direction" : <"ascending"|"descending">,
"dimensionOrder" : <"lexicographic"(default)|"alphanumeric"|"strlen"|"numeric">
}
```
If only the dimension is provided (as a JSON string), the default order-by is ascending with lexicographic sorting.
See [Sorting Orders](./sorting-orders.md) for more information on the sorting orders specified by "dimensionOrder".