clintropolis opened a new pull request #11241:
URL: https://github.com/apache/druid/pull/11241
### Description
This PR adds `STRING_AGG`, which is sort of like `ARRAY_AGG` added in #11157
but it ignores null values, with an `ARRAY_TO_STRING` function on the output
array to turn it into a string value.
example:
```sql
SELECT STRING_AGG(l1, '-'), STRING_AGG(DISTINCT l1, ',') FROM numfoo
```
output:
```
["7-325323-0-0-0-0", "0,7,325323"]
```
|Function|Notes|Default|
|--------|-----|-------|
|`STRING_AGG(expr, separator, [size])`|Collects all values of `expr` into a
single STRING, ignoring null values. Each value is joined by the `separator`
which must be a literal STRING. An optional `size` in bytes can be supplied to
limit aggregation size (default of 1024 bytes). If the aggregated string grows
larger than the maximum size in bytes, the query will fail. Use of `ORDER BY`
within the `STRING_AGG` expression is not currently supported, and the ordering
of results within the output string may vary depending on processing
order.|`null` if `druid.generic.useDefaultValueForNull=false`, otherwise `''`|
|`STRING_AGG(DISTINCT expr, separator, [size])`|Collects all distinct values
of `expr` into a single STRING, ignoring null values. Each value is joined by
the `separator` which must be a literal STRING. An optional `size` in bytes can
be supplied to limit aggregation size (default of 1024 bytes). If the
aggregated string grows larger than the maximum size in bytes, the query will
fail. Use of `ORDER BY` within the `STRING_AGG` expression is not currently
supported, and the ordering of results within the output string may vary
depending on processing order.|`null` if
`druid.generic.useDefaultValueForNull=false`, otherwise `''`|
Like `ARRAY_AGG` this is an expression aggregator, so is not well optimized,
but will fill in the gaps for now.
This PR has:
- [ ] been self-reviewed.
- [ ] using the [concurrency
checklist](https://github.com/apache/druid/blob/master/dev/code-review/concurrency.md)
(Remove this item if the PR doesn't have any relation to concurrency.)
- [ ] added documentation for new or modified features or behaviors.
- [ ] added Javadocs for most classes and all non-trivial methods. Linked
related entities via Javadoc links.
- [ ] added or updated version, license, or notice information in
[licenses.yaml](https://github.com/apache/druid/blob/master/dev/license.md)
- [ ] added comments explaining the "why" and the intent of the code
wherever would not be obvious for an unfamiliar reader.
- [ ] added unit tests or modified existing tests to cover new code paths,
ensuring the threshold for [code
coverage](https://github.com/apache/druid/blob/master/dev/code-review/code-coverage.md)
is met.
- [ ] added integration tests.
- [ ] been tested in a test Druid cluster.
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