asdf2014 commented on code in PR #16516: URL: https://github.com/apache/druid/pull/16516#discussion_r1634308041
########## docs/release-info/migr-mvd-array.md: ########## @@ -0,0 +1,245 @@ +--- +id: migr-mvd-array +title: "Migration guide: MVDs to arrays" +sidebar_label: MVDs to arrays +--- + +<!-- + ~ 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. +--> + + +Druid now supports SQL-compliant [arrays](../querying/arrays.md), and we recommend that people use arrays over [multi-value dimensions](../querying/multi-value-dimensions.md) (MVDs) whenever possible. +Use arrays for new projects and complex use cases involving multiple data types. Use MVDs for specific use cases, such as operating directly on individual elements like regular strings. If your operations involve complete arrays of values, including the ordering of values within a row, use arrays over MVDs. + +## Comparison between arrays and MVDs + +The following table compares the general behavior between arrays and MVDs. +For specific query differences between arrays and MVDs, see [Querying arrays and MVDs](#querying-arrays-and-mvds). + +| | Arrays| Multi-value dimensions (MVDs) | +|---|---|---| +| Data types | Supports VARCHAR, BIGINT, and DOUBLE types (ARRAY<STRING\>, ARRAY<LONG\>, ARRAY<DOUBLE\>) | Only supports arrays of strings (VARCHAR) | +| SQL compliance | Behaves like standard SQL arrays with SQL-compliant behavior | Behaves like SQL VARCHAR rather than standard SQL arrays and requires special SQL functions to achieve array-like behavior. See the [examples](#examples). | +| Ingestion | <ul><li>JSON arrays are ingested as Druid arrays</li><li>Managed through the query context parameter `arrayIngestMode` in SQL-based ingestion (supported options: `array`, `mvd`, `none`). Note that if you set this mode to `none`, Druid raises an exception if you try to store any type of array.</li></ul> | <ul><li>JSON arrays are ingested as multi-value dimensions</li><li>Managed using functions like [ARRAY_TO_MV](../querying/sql-functions.md#array_to_mv) in SQL-based ingestion</li></ul> | +| Filtering and grouping | <ul><li>Filters and groupings match the entire array value</li><li>Can be used as GROUP BY keys, grouping based on the entire array value</li></ul> | <ul><li>Filters match any value within the array</li><li>Grouping generates a group for each individual value, similar to an implicit UNNEST</li></ul> | +| Conversion | Convert an MVD to an array using [MV_TO_ARRAY](../querying/sql-functions.md#mv_to_array) | Convert an array to an MVD using [ARRAY_TO_MV](../querying/sql-functions.md#array_to_mv) | + +## Querying arrays and MVDs + +In SQL queries, Druid operates on arrays differently than MVDs. +A value in an array column is treated as a single array entity (SQL ARRAY), whereas a value in an MVD column is treated as individual strings (SQL VARCHAR). +This behavior applies even though multiple string values within the same MVD are still stored as a single field in the MVD column. + +For example, consider the same value, `['a', 'b', 'c']` ingested into an array column and an MVD column. +In your query, you want to filter results by comparing some value with `['a', 'b', 'c']`. + +* For array columns, Druid only returns the row when an equality filter matches the entire array. +For example: `WHERE "array_column" = ARRAY['a', 'b', 'c']`. + +* For MVD columns, Druid returns the row when an equality filter matches any value of the MVD. +For example, any of the following filters returns the row for the query: +`WHERE "mvd_column" = 'a'` +`WHERE "mvd_column" = 'b'` +`WHERE "mvd_column" = 'c'` + +Note this difference between arrays and MVDs when you write queries that involve filtering or grouping. +When your query applies both filters and grouping, MVDs may return rows that don't seem to match the filter, +since the grouping occurs after Druid performs the filter. For an example, see [Filter and group by array elements](#filter-and-group-by-array-elements). + +## Examples + +The following examples highlight a few analogous queries between arrays and MVDs. +For more information and examples, see [Querying arrays](../querying/arrays.md#querying-arrays) and [Querying multi-value dimensions](../querying/multi-value-dimensions.md#querying-multi-value-dimensions). + +### Filter by an array element + +Filter rows that have a certain value in the array or MVD. + +#### Array + +```sql +SELECT label, tags +FROM "array_example" +WHERE ARRAY_CONTAINS(tags, 't3') +``` + +#### MVD + +```sql +SELECT label tags, +FROM "mvd_example" +WHERE tags = 't3' +``` + +### Filter by one or more elements + +Filter rows for which the array or MVD contains one or more elements. +Notice that [ARRAY_OVERLAP](../querying/sql-functions.md#array_overlap) checks for any overlapping elements, whereas [ARRAY_CONTAINS](../querying/sql-functions.md#array_contains) in the previous example checks that all elements are included. + +#### Array + +```sql +SELECT * +FROM "array_example" +WHERE ARRAY_OVERLAP(tags, ARRAY['t1', 't7']) +``` + +#### MVD + +```sql +SELECT * +FROM "mvd_example" +WHERE tags = 't1' OR tags = 't7' +``` + +### Filter using array equality + +Filters rows for which the array or MVD is equivalent to a reference array. + +#### Array + +```sql +SELECT * +FROM "array_example" +WHERE tags = ARRAY['t1', 't2', 't3'] +``` + +#### MVD + +```sql +SELECT * +FROM "mvd_example" +WHERE MV_TO_ARRAY(tags) = ARRAY['t1', 't2', 't3'] +``` + +### Group results by array + +Groups results by the array or MVD. + +#### Array + +```sql +SELECT label, tags +FROM "array_example" +GROUP BY 1, 2 +``` + +#### MVD + +```sql +SELECT label, MV_TO_ARRAY(tags) +FROM "mvd_example" +GROUP BY 1, 2 +``` + +### Group by array elements + +Group results by individual array or MVD elements. + +#### Array + +```sql +SELECT label, strings +FROM "array_example" CROSS JOIN UNNEST(tags) as u(strings) +GROUP BY 1, 2 +``` + +#### MVD + +```sql +SELECT label, tags +FROM "mvd_example" +GROUP BY 1, 2 +``` + +### Filter and group by array elements + +Filter rows that have a certain value, then group by elements in the array or MVD. +This example illustrates that while the the results of filtering may match between arrays and MVDs, Review Comment: ```suggestion This example illustrates that while the results of filtering may match between arrays and MVDs, ``` -- This is an automated message from the Apache Git Service. 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