fix unit tests by handling nulls

nulls were getting greatest()'d during materialize

Author: dlawin

data_diff/databases/postgresql.py

@@ -102,7 +102,14 @@ def md5_as_hex(self, s: str) -> str:
         return f"md5({s})"
 
     def normalize_timestamp(self, value: str, coltype: TemporalType) -> str:
+        def _add_padding(coltype: TemporalType, timestamp6: str):
+            return f"RPAD(LEFT({timestamp6}, {TIMESTAMP_PRECISION_POS+coltype.precision}), {TIMESTAMP_PRECISION_POS+6}, '0')"
+
         if coltype.rounds:
+            # NULL value expected to return NULL after normalization
+            null_case_begin = f"CASE WHEN {value} IS NULL THEN NULL ELSE "
+            null_case_end = "END"
+
             # 294277 or 4714 BC would be out of range, make sure we can't round to that
             # TODO test timezones for overflow?
             max_timestamp = "294276-12-31 23:59:59.0000"
@@ -112,20 +119,23 @@ def normalize_timestamp(self, value: str, coltype: TemporalType) -> str:
 
             interval = format((0.5 * (10 ** (-coltype.precision))), f".{coltype.precision+1}f")
 
-            final_timestamp = (
+            rounded_timestamp = (
                 f"left(to_char(least('{max_timestamp}'::timestamp, {timestamp})"
                 f"+ interval '{interval}', 'YYYY-mm-dd HH24:MI:SS.US'),"
                 f"length(to_char(least('{max_timestamp}'::timestamp, {timestamp})"
                 f"+ interval '{interval}', 'YYYY-mm-dd HH24:MI:SS.US')) - (6-{coltype.precision}))"
             )
 
+            padded = _add_padding(coltype, rounded_timestamp)
+            return f"{null_case_begin} {padded} {null_case_end}"
+
             # TODO years with > 4 digits not padded correctly
             # current w/ precision 6: 294276-12-31 23:59:59.0000
             # should be 294276-12-31 23:59:59.000000
         else:
-            final_timestamp = f"to_char({value}::timestamp(6), 'YYYY-mm-dd HH24:MI:SS.US')"
-
-        return f"RPAD(LEFT({final_timestamp}, {TIMESTAMP_PRECISION_POS+coltype.precision}), {TIMESTAMP_PRECISION_POS+6}, '0')"
+            rounded_timestamp = f"to_char({value}::timestamp(6), 'YYYY-mm-dd HH24:MI:SS.US')"
+            padded = _add_padding(coltype, rounded_timestamp)
+            return padded
 
     def normalize_number(self, value: str, coltype: FractionalType) -> str:
         return self.to_string(f"{value}::decimal(38, {coltype.precision})")