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docs: name the invalid UTF-8 cases that 1.1.0 kept in Spark #7121

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19 changes: 17 additions & 2 deletions docs/source/user-guide/latest/compatibility/index.md
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Expand Up @@ -98,8 +98,23 @@ underlying bytes rather than on the rendered text:
such as `contains` can therefore disagree with Spark when non-UTF-8 bytes are involved.

Both differences require string data that is not valid UTF-8, which does not occur for text read from
Parquet or produced by string expressions. Consistent handling of invalid UTF-8 across all native
string paths is tracked by
Parquet or produced by string expressions. It can occur in the following cases, where a query that
ran entirely in Spark in Comet 1.1.0 can return different results:

- **Text sources on a broadcast build side.** When a Text scan, which Comet cannot read natively,
feeds the build side of a broadcast join, Comet converts it to Arrow so the join runs natively
(see [Data Sources](../datasources.md)). A Text file is the source most likely to hold bytes that are
not valid UTF-8, for example a Latin-1 file, so `hex`, `length` and `octet_length` of its string
columns can differ from Spark: the byte `0xE9` becomes the three bytes `EFBFBD`. Set
`spark.comet.convert.broadcastBuildSide.enabled=false` to keep such build sides on Spark.
- **`min` and `max` over a string cast from binary.** The cast decodes the bytes before the
comparison, so the ordering can differ from Spark and the minimum and maximum can swap. For
example, over the values `CAST(X'80' AS STRING)` and `CAST(X'C3A9' AS STRING)`, Spark returns the
`0x80` string as the minimum and `é` as the maximum, whereas Comet returns `é` as the minimum and
`U+FFFD` as the maximum. Aggregate over the `BINARY` column instead, or disable Comet for the query,
if the data may hold bytes that are not valid UTF-8.

Consistent handling of invalid UTF-8 across all native string paths is tracked by
[#4764](https://github.com/apache/datafusion-comet/issues/4764).

Separately, Comet's native Parquet scan currently rejects string columns whose stored bytes are not
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6 changes: 4 additions & 2 deletions docs/source/user-guide/latest/datasources.md
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Expand Up @@ -78,8 +78,10 @@ runs natively over the large probe. This is controlled by
`spark.comet.convert.broadcastBuildSide.enabled`, which is **on by default**, unlike the switches
above. The build side is usually small, but an explicit `BROADCAST` hint or a selective filter above
the scan can make the copied volume larger, and with dynamic partition pruning the dimension is read
a second time for the pruning subquery. Set `spark.comet.convert.broadcastBuildSide.enabled=false` to
keep such build sides on Spark.
a second time for the pruning subquery. A Text source can also hold bytes that are not valid UTF-8,
which Comet replaces with `U+FFFD` during the conversion (see
[Strings with non-UTF-8 bytes](compatibility/index.md#strings-with-non-utf-8-bytes)). Set
`spark.comet.convert.broadcastBuildSide.enabled=false` to keep such build sides on Spark.

### Spark-to-Comet conversion types

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