[GitHub] [arrow-datafusion] alamb commented on a diff in pull request #7042: Minor: add test + docs for 2 argument trunc with columns

["alamb (via GitHub)" <gi...@apache.org>]

alamb commented on code in PR #7042:
URL: https://github.com/apache/arrow-datafusion/pull/7042#discussion_r1275382805


##########
datafusion/core/tests/sqllogictests/test_files/scalar.slt:
##########
@@ -918,6 +918,15 @@ select trunc(4.267, 3), trunc(1.1234, 2), trunc(-1.1231, 6), trunc(1.2837284, 2)
 ----
 4.267 1.12 -1.1231 1.28 1
 
+# trunc with columns and precision
+query RRR rowsort
+select trunc(a, 3) as a3, trunc(a, 1) as a1, trunc(arrow_cast(a, 'Float64'), 3) as a3_f64 from small_floats;
+----
+-0.7 -0.7 -0.7
+-1 -1 -1
+0.2 0.2 0.2
+0.5 0.5 0.5

Review Comment:
   Added



##########
docs/source/user-guide/sql/scalar_functions.md:
##########
@@ -507,14 +507,17 @@ tanh(numeric_expression)
 Truncates a number toward zero (at the decimal point).
 
 ```
-trunc(numeric_expression)
+trunc(numeric_expression[, decimal_places])
 ```
 
 #### Arguments
 
 - **numeric_expression**: Numeric expression to operate on.
   Can be a constant, column, or function, and any combination of arithmetic operators.
 
+- **decimal_places**: Optional. The number of decimal places to truncate to.
+  Defaults to 0.

Review Comment:
   TIL turns out that negative precision means "truncate to the left of the decimal place": https://www.postgresqltutorial.com/postgresql-math-functions/postgresql-trunc/#:~:text=If%20the%20precision%20argument%20is,The%20precision%20argument%20is%20optional. 
   
   ```
   postgres=# select trunc(12345.6789, -2);
    trunc
   -------
    12300
   (1 row)
   ```
   
   This is actually what the current datafusion implementation does too so I have added a test and updated the docs
   



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