Github user youngbink commented on the issue:
https://github.com/apache/spark/pull/20015
@HyukjinKwon Just took a look at this PR #14788.
My point of mentioning those databases was just to give examples of the
function that Spark doesn't support but other databases commonly do. (They all
have this `date_trunc` which takes `timestamp` and output `timestamp`)
As you said, we could extend `trunc` and simply create an alias
`date_trunc`, but it's actually not as simple. For e.g, PR #14788 won't be able
to handle the following command collectly on PySpark:
```
df = spark.createDataFrame([('1997-02-28 05:02:11',)], ['d'])
df.select(functions.trunc(df.d, 'year').alias('year')).collect()
df.select(functions.trunc(df.d, 'SS').alias('SS')).collect()
```
This is because `trunc(string, string)` isn't correctly handled. We could
find a way around this and get it working, but after having a discussion with
@cloud-fan, @gatorsmile, @rednaxelafx and Reynold, we decided to add
`date_trunc` to be compatible with Postgres for now.
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