mirror of https://github.com/apache/druid.git
71 lines
3.1 KiB
Markdown
71 lines
3.1 KiB
Markdown
---
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id: limitspec
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title: "Sorting and limiting (groupBy)"
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---
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~ Licensed to the Apache Software Foundation (ASF) under one
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~ or more contributor license agreements. See the NOTICE file
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> Apache Druid supports two query languages: [Druid SQL](sql.md) and [native queries](querying.md).
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> This document describes the native
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> language. For information about sorting in SQL, refer to the [SQL documentation](sql-syntax.md#order-by).
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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:
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### DefaultLimitSpec
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The default limit spec takes a limit and the list of columns to do an orderBy operation over. The grammar is:
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```json
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{
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"type" : "default",
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"limit" : <optional integer>,
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"offset" : <optional integer>,
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"columns" : [<optional list of OrderByColumnSpec>],
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}
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```
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The "limit" parameter is the maximum number of rows to return.
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The "offset" parameter tells Druid to skip this many rows when returning results. If both "limit" and "offset" are
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provided, then "offset" will be applied first, followed by "limit". For example, a spec with limit 100 and offset 10
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will return 100 rows starting from row number 10. Internally, the query is executed by extending the limit by the offset
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and then discarding a number of rows equal to the offset. This means that raising the offset will increase resource
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usage by an amount similar to increasing the limit.
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Together, "limit" and "offset" can be used to implement pagination. However, note that if the underlying datasource is
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modified in between page fetches in ways that affect overall query results, then the different pages will not
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necessarily align with each other.
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#### OrderByColumnSpec
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OrderByColumnSpecs indicate how to do order by operations. Each order-by condition can be a `jsonString` or a map of the following form:
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```json
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{
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"dimension" : "<Any dimension or metric name>",
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"direction" : <"ascending"|"descending">,
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"dimensionOrder" : <"lexicographic"(default)|"alphanumeric"|"strlen"|"numeric">
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}
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```
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If only the dimension is provided (as a JSON string), the default order-by is ascending with lexicographic sorting.
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See [Sorting Orders](./sorting-orders.md) for more information on the sorting orders specified by "dimensionOrder".
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