clintropolis commented on a change in pull request #11188:
URL: https://github.com/apache/druid/pull/11188#discussion_r628489278
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File path: docs/querying/sql.md
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@@ -293,15 +293,15 @@ the `UNNEST` functionality available in some other SQL
dialects. Refer to the do
The `druid.generic.useDefaultValueForNull` [runtime
property](../configuration/index.md#sql-compatible-null-handling)
controls Druid's NULL handling mode.
-In the default mode (`true`), Druid treats NULLs and empty strings
interchangeably, rather than according to the SQL
+In the default value mode (`true`), Druid treats NULLs and empty strings
interchangeably, rather than according to the SQL
standard. In this mode Druid SQL only has partial support for NULLs. For
example, the expressions `col IS NULL` and
`col = ''` are equivalent, and both will evaluate to true if `col` contains an
empty string. Similarly, the expression
`COALESCE(col1, col2)` will return `col2` if `col1` is an empty string. While
the `COUNT(*)` aggregator counts all rows,
the `COUNT(expr)` aggregator will count the number of rows where expr is
neither null nor the empty string. Numeric
columns in this mode are not nullable; any null or missing values will be
treated as zeroes.
In SQL compatible mode (`false`), NULLs are treated more closely to the SQL
standard. The property affects both storage
-and querying, so for best behavior, it should be set at both ingestion time
and query time. There is some overhead
+and querying, so for correct behavior, it should be set on all Druid service
types to be available at both ingestion time and query time. There is some
overhead
associated with the ability to handle NULLs; see the [segment
internals](../design/segments.md#sql-compatible-null-handling)
Review comment:
hmm, we're going to need to watch out for this for a while in any PR
that is adding these SQL tests that use this context parameter.
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