{"action":"created","comment":{"url":"https://api.github.com/repos/druid-io/druid/pulls/comments/35463188","id":35463188,"diff_hunk":"@@ -0,0 +1,50 @@\n+---\n+layout: doc_page\n+---\n+\n+# Schema Design\n+\n+This page is meant to assist users in designing a schema for data to be ingested in Druid. Druid intakes denormalized data \n+and columns are one of three types: a timestamp column, a dimension, or a measure (or a metric/aggregator as they are \n+known in Druid). This follows the [standard naming convention](https://en.wikipedia.org/wiki/Online_analytical_processing#Overview_of_OLAP_systems) \n+of OLAP data.\n+\n+Typical production tables (or datasources as they are known in Druid) have less than 100 dimensions and less \n+than 100 metrics, although, based on user testimony, datasources with thousands of dimensions have been created. At the \n+time of writing, we have not benchmarked the upper limits of how many columns Druid is capable of.\n+\n+Dimension values in Druid can either be a single valued String, or an array of String values. Metric columns can be \n+integers, longs, floats, or in a user defined 'complex' format. Metric values may be [rolled-up](../design/index.html) \n+data at ingestion time. \n+\n+Below, we outline some best practices with schema design:\n+\n+## High cardinality dimensions (e.g. unique IDs)\n+\n+In practice, we see that exact counts for unique IDs are often not required. Storing unique IDs as a column will kill \n+roll-up ratios, and impact compression. Instead, storing a sketch of the number of the unique IDs seen, and using that 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example of 1) indexing spec, 2) query would help illustrate how to do this","created_at":"2015-07-24T20:35:53Z","updated_at":"2015-07-24T20:35:53Z","html_url":"https://github.com/druid-io/druid/pull/1553#discussion_r35463188","pull_request_url":"https://api.github.com/repos/druid-io/druid/pulls/1553","_links":{"self":{"href":"https://api.github.com/repos/druid-io/druid/pulls/comments/35463188"},"html":{"href":"https://github.com/druid-io/druid/pull/1553#discussion_r35463188"},"pull_request":{"href":"https://api.github.com/repos/druid-io/druid/pulls/1553"}}},"pull_request":{"url":"https://api.github.com/repos/druid-io/druid/pulls/1553","id":40805274,"html_url":"https://github.com/druid-io/druid/pull/1553","diff_url":"https://github.com/druid-io/druid/pull/1553.diff","patch_url":"https://github.com/druid-io/druid/pull/1553.patch","issue_url":"https://api.github.com/repos/druid-io/druid/issues/1553","number":1553,"state":"open","locked":false,"title":"Added section on best practices for 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