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* Refresh query docs. Larger changes: - New doc: querying/datasource.md describes the various kinds of datasources you can use, and has examples for both SQL and native. - New doc: querying/query-execution.md describes how native queries are executed at a high level. It doesn't go into the details of specific query engines or how queries run at a per-segment level. But I think it would be good to add or link that content here in the future. - Refreshed doc: querying/sql.md updated to refer to joins, reformatted a bit, added a new "Query translation" section that explains how queries are translated from SQL to native, and removed configuration details (moved to configuration/index.md). - Refreshed doc: querying/joins.md updated to refer to join datasources. Smaller changes: - Add helpful banners to the top of query documentation pages telling people whether a given page describes SQL, native, or both. - Add SQL metrics to operations/metrics.md. - Add some color and cross-links in various places. - Add native query component docs to the sidebar, and renamed them so they look nicer. - Remove Select query from the sidebar. - Fix Broker SQL configs in configuration/index.md. Remove them from querying/sql.md. - Combined querying/searchquery.md and querying/searchqueryspec.md. * Updates. * Fix numbering. * Fix glitches. * Add new words to spellcheck file. * Assorted changes. * Further adjustments. * Add missing punctuation.
41 lines
2.0 KiB
Markdown
41 lines
2.0 KiB
Markdown
---
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id: joins
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title: "Joins"
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---
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Druid has two features related to joining of data:
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1. [Join](datasource.md#join) operators. These are available using a [join datasource](datasource.md#join) in native
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queries, or using the [JOIN operator](sql.md#query-syntax) in Druid SQL. Refer to the
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[join datasource](datasource.md#join) documentation for information about how joins work in Druid.
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2. [Query-time lookups](lookups.md), simple key-to-value mappings. These are preloaded on all servers that are involved
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in queries and can be accessed with or without an explicit join operator. Refer to the [lookups](lookups.md)
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documentation for more details.
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Whenever possible, for best performance it is good to avoid joins at query time. Often this can be accomplished by
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joining data before it is loaded into Druid. However, there are situations where joins or lookups are the best solution
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available despite the performance overhead, including:
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- The fact-to-dimension (star and snowflake schema) case: you need to change dimension values after initial ingestion,
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and aren't able to reingest to do this. In this case, you can use lookups for your dimension tables.
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- Your workload requires joins or filters on subqueries.
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