Convert string inputs to timestamp when fed to unix_timestamp function

Signed-off-by: Yuanchun Shen <yuanchu@amazon.com>

Author: yuancu

core/src/main/java/org/opensearch/sql/expression/datetime/DateTimeFunctions.java

@@ -2164,14 +2164,21 @@ public static Double transferUnixTimeStampFromDoubleInput(Double value) {
     }
   }
 
+  private static double getUnixTimestampAsDouble(ExprTimestampValue value) {
+    return value.timestampValue().getEpochSecond() + value.timestampValue().getNano() / 1E9;
+  }
+
   private Double unixTimeStampOfImpl(ExprValue value) {
     // Also, according to MySQL documentation:
     //    The date argument may be a DATE, DATETIME, or TIMESTAMP ...
     switch ((ExprCoreType) value.type()) {
       case DATE:
         return value.dateValue().toEpochSecond(LocalTime.MIN, ZoneOffset.UTC) + 0d;
       case TIMESTAMP:
-        return value.timestampValue().getEpochSecond() + value.timestampValue().getNano() / 1E9;
+        return getUnixTimestampAsDouble((ExprTimestampValue) value);
+      case STRING:
+        ExprTimestampValue timestampValue = new ExprTimestampValue(value.stringValue());
+        return getUnixTimestampAsDouble(timestampValue);
       default:
         //     ... or a number in YYMMDD, YYMMDDhhmmss, YYYYMMDD, or YYYYMMDDhhmmss format.
         //     If the argument includes a time part, it may optionally include a fractional

docs/user/ppl/functions/datetime.rst

@@ -1899,7 +1899,7 @@ The date argument may be a DATE, or TIMESTAMP string, or a number in YYMMDD, YYM
 If argument is in invalid format or outside of range 1970-01-01 00:00:00 - 3001-01-18 23:59:59.999999 (0 to 32536771199.999999 epoch time), function returns NULL.
 You can use `FROM_UNIXTIME`_ to do reverse conversion.
 
-Argument type: <NONE>/DOUBLE/DATE/TIMESTAMP
+Argument type: <NONE>/DOUBLE/DATE/TIMESTAMP/STRING
 
 Return type: DOUBLE
 

integ-test/src/test/java/org/opensearch/sql/calcite/standalone/CalcitePPLDateTimeBuiltinFunctionIT.java

@@ -383,6 +383,18 @@ public void testUnixTimestamp() {
     verifyDataRows(actual, closeTo(1220249547.0, 3404817525.0, 1.220249547123456E9));
   }
 
+  @Test
+  public void testUnixTimestampWithString() {
+    JSONObject actual =
+        executeQuery(
+            String.format(
+                "source=%s | head 1 | eval t = UNIX_TIMESTAMP('2008-09-01 06:12:27.123456') |"
+                    + " fields t",
+                TEST_INDEX_DATE_FORMATS));
+    verifySchema(actual, schema("t", "double"));
+    verifyDataRows(actual, closeTo(1.220249547123456E9));
+  }
+
   @Test
   public void testWeekAndWeekOfYear() {
     JSONObject actual =