Support yyyyMMdd in GetTimestamp operator for LEGACY mode [databricks]

docs/compatibility.md

@@ -651,6 +651,7 @@ guaranteed to produce the same results as the CPU:
 - `dd/MM/yyyy`
 - `yyyy/MM/dd`
 - `yyyy-MM-dd`
+- `yyyyMMdd`
 - `yyyy/MM/dd HH:mm:ss`
 - `yyyy-MM-dd HH:mm:ss`
 
@@ -659,6 +660,8 @@ LEGACY timeParserPolicy support has the following limitations when running on th
 - Only 4 digit years are supported
 - The proleptic Gregorian calendar is used instead of the hybrid Julian+Gregorian calendar
   that Spark uses in legacy mode
+- When format is `yyyyMMdd`, GPU only supports 8 digit strings. Spark supports like 7 digit
+  `2024101` string while GPU does not support.
 
 ## Formatting dates and timestamps as strings
 

integration_tests/src/main/python/date_time_test.py

@@ -13,7 +13,7 @@
 # limitations under the License.
 
 import pytest
-from asserts import assert_gpu_and_cpu_are_equal_collect, assert_gpu_fallback_collect, assert_gpu_and_cpu_error
+from asserts import assert_gpu_and_cpu_are_equal_collect, assert_gpu_fallback_collect, assert_gpu_and_cpu_error, assert_gpu_and_cpu_are_equal_sql
 from conftest import is_utc, is_supported_time_zone, get_test_tz
 from data_gen import *
 from datetime import date, datetime, timezone
@@ -460,6 +460,15 @@ def test_to_timestamp(parser_policy):
         { "spark.sql.legacy.timeParserPolicy": parser_policy})
 
 
+def test_to_timestamp_legacy_mode_yyyyMMdd_format():
+    gen = StringGen("[0-9]{3}[1-9](0[1-9]|1[0-2])(0[1-9]|[1-2][0-9])")
+    assert_gpu_and_cpu_are_equal_collect(
+        lambda spark : unary_op_df(spark, gen).select(f.to_timestamp(f.col("a"), "yyyyMMdd")),
+        {
+                'spark.sql.legacy.timeParserPolicy': 'LEGACY',
+                'spark.rapids.sql.incompatibleDateFormats.enabled': True})
+
+
 @tz_sensitive_test
 @pytest.mark.skipif(not is_supported_time_zone(), reason="not all time zones are supported now, refer to https://github.com/NVIDIA/spark-rapids/issues/6839, please update after all time zones are supported")
 @pytest.mark.parametrize("ansi_enabled", [True, False], ids=['ANSI_ON', 'ANSI_OFF'])

sql-plugin/src/main/scala/org/apache/spark/sql/rapids/datetimeExpressions.scala

@@ -640,7 +640,9 @@ object GpuToTimestamp {
     "yyyy-MM-dd HH:mm:ss" -> ParseFormatMeta(Option('-'), isTimestamp = true,
       raw"\A\d{4}-\d{1,2}-\d{1,2}[ T]\d{1,2}:\d{1,2}:\d{1,2}(\D|\s|\Z)"),
     "yyyy/MM/dd HH:mm:ss" -> ParseFormatMeta(Option('/'), isTimestamp = true,
-      raw"\A\d{4}/\d{1,2}/\d{1,2}[ T]\d{1,2}:\d{1,2}:\d{1,2}(\D|\s|\Z)")
+      raw"\A\d{4}/\d{1,2}/\d{1,2}[ T]\d{1,2}:\d{1,2}:\d{1,2}(\D|\s|\Z)"),
+    "yyyyMMdd" -> ParseFormatMeta(None, isTimestamp = false,
+      raw"\A\d{8}(\D|\s|\Z)")
   )
 
   /** remove whitespace before month and day */
@@ -762,8 +764,21 @@ object GpuToTimestamp {
           case RegexReplace(pattern, backref) =>
             RegexReplace(pattern.replace('-', '/'), backref.replace('-', '/'))
         }
-      case Some('-') | Some(_) | None =>
+      case Some('-') | Some(_) =>
         regexReplaceRules
+      case None =>
+        // For formats like `yyyyMMdd` that do not contains separator,
+        // do not need to do regexp replacement rules
+        // Note: here introduced the following inconsistent behavior compared to Spark
+        // Spark's behavior:
+        //   to_date('20240101', 'yyyyMMdd')  = 2024-01-01
+        //   to_date('202401 01', 'yyyyMMdd') = 2024-01-01
+        //   to_date('2024 0101', 'yyyyMMdd') = null
+        // GPU behavior:
+        //   to_date('20240101', 'yyyyMMdd')  = 2024-01-01
+        //   to_date('202401 01', 'yyyyMMdd') = null
+        //   to_date('2024 0101', 'yyyyMMdd') = null
+        Seq()
     }
 
     // apply each rule in turn to the data