[DOC] Misleading documentation for spark.rapids.sql.incompatibleDateFormats.enabled

[andygrove]

Report incorrect documentation

Location of incorrect documentation

The RapidsConf documentation for spark.rapids.sql.incompatibleDateFormats.enabled is slightly misleading.

Describe the problems or issues found in the documentation

The documentation states that:

When parsing strings as dates and timestamps in functions like unix_timestamp, 
setting this to true will force all parsing onto GPU even for formats that can 
result in incorrect results when parsing invalid inputs.

What isn't clear here is that some formats are not supported at all on GPU and will still fall back to CPU. For example, we don't support formats that include MMM on GPU.

Steps taken to verify documentation is incorrect

scala> spark.conf.set("spark.sql.session.timeZone", "UTC")
scala> spark.conf.set("spark.rapids.sql.incompatibleDateFormats.enabled", "true")
scala> val df_notsupported = Seq(("2021-Dec-25 11:11:11")).toDF("ts")
scala> df_notsupported.write.format("parquet").mode("overwrite").save("/tmp/testts_notsupported.parquet")
scala> spark.read.parquet("/tmp/testts_notsupported.parquet").createOrReplaceTempView("df_notsupported")
scala> val df = spark.sql("select to_timestamp(ts, 'yyyy-MMM-dd HH:mm:ss') from df_notsupported")
df: org.apache.spark.sql.DataFrame = [to_timestamp(ts, yyyy-MMM-dd HH:mm:ss): timestamp]

scala> df.collect
21/03/23 12:46:29 WARN GpuOverrides: 
!Exec <ProjectExec> cannot run on GPU because unsupported data types in output: TimestampType; not all expressions can be replaced
  !Expression <Alias> gettimestamp(ts#7, yyyy-MMM-dd HH:mm:ss, Some(UTC), true) AS to_timestamp(ts, yyyy-MMM-dd HH:mm:ss)#9 cannot run on GPU because expression GetTimestamp gettimestamp(ts#7, yyyy-MMM-dd HH:mm:ss, Some(UTC), true) produces an unsupported type TimestampType; expression Alias gettimestamp(ts#7, yyyy-MMM-dd HH:mm:ss, Some(UTC), true) AS to_timestamp(ts, yyyy-MMM-dd HH:mm:ss)#9 produces an unsupported type TimestampType
    !Expression <GetTimestamp> gettimestamp(ts#7, yyyy-MMM-dd HH:mm:ss, Some(UTC), true) cannot run on GPU because expression GetTimestamp gettimestamp(ts#7, yyyy-MMM-dd HH:mm:ss, Some(UTC), true) produces an unsupported type TimestampType; Failed to convert Unsupported word: MMM null
      @Expression <AttributeReference> ts#7 could run on GPU
      @Expression <Literal> yyyy-MMM-dd HH:mm:ss could run on GPU
  *Exec <FileSourceScanExec> will run on GPU

Suggested fix for documentation

When parsing strings as dates and timestamps in functions like unix_timestamp, some formats are
fully supported on GPU, some are supported but can produce incorrect results for invalid inputs, 
and others are not supported at all. Setting this to true will force all parsing onto GPU for supported formats, 
including formats that can result in incorrect results.
```