vtlim commented on code in PR #16516: URL: https://github.com/apache/druid/pull/16516#discussion_r1626639683
########## docs/release-info/migr-mvd-array.md: ########## @@ -0,0 +1,68 @@ +--- +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 arrays of strings. + +## 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 [Query differences between arrays and MVDs](#query-differences-between-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 | Does not behave like standard SQL arrays; requires special SQL functions | +| Ingestion | <ul><li>Can be ingested using native batch or streaming ingestion methods</li><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>Typically ingested from fields with an array of values</li><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-multivalue-string-functions.md) | Convert an array to an MVD using [ARRAY_TO_MV](../querying/sql-functions.md#array_to_mv) | + +### Query differences between arrays and MVDs + +Querying an array column returns different results than when querying a MVD column depending on the query type. Review the following table on the differences. Adjust your applications to handle the new output shapes. + +| Query type | Array | MVD | +|---|---|---| +| Equality filter | Matches the entire array value | Matches if any value within the array matches the filter | +| Null filter | Matches rows where the entire array value is null | Matches rows where the array is empty (considered as null) but does not match arrays with empty (`“”`) values | +| Range filter | Use [ARRAY_OVERLAP](../querying/sql-functions.md#array_overlap) | Not directly supported | +| Contains filter | Use [ARRAY_CONTAINS](../querying/sql-functions.md#array_contains)| Use WHERE filter | +| Logical expression filters | Behaves like standard ANSI SQL on the entire array value, such as AND, OR, NOT. For example, `WHERE arrayLong = ARRAY[1,2,3] OR arrayLong = ARRAY[4,5,6]` | Matches a row if any value within the array matches the logical condition. For example, `WHERE tags = 't1' OR tags = 't3'` | +| Column comparison filter | Use [ARRAY_OVERLAP](../querying/sql-functions.md#array_overlap) | Matches when the dimensions have any overlapping values. For example, `WHERE tags IN ('t1', 't2')` | +| Behavior with SQL constructs | Follows standard SQL behavior with array functions like [ARRAY_CONTAINS](../querying/sql-functions.md#array_contains), [ARRAY_OVERLAP](../querying/sql-functions.md#array_overlap) | Requires special SQL functions like [MV_FILTER_ONLY](../querying/sql-functions.md#mv_filter_none), [MV_FILTER_NONE](../querying/sql-functions.md#mv_filter_only) for precise filtering | +| Group by entire array | Groups the entire array as a single value | Not supported | +| Group by individual values | Use [UNNEST](../querying/sql.md#unnest) to group by individual array elements | Automatically unnests groups by each individual value in the array | + +## How to ingest data as arrays + Review Comment: linking out to external docs for examples and more details -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. 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