Prioritize the signature using timestamp for unix_timestamp function

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

Author: yuancu

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

@@ -981,8 +981,8 @@ private FunctionResolver unix_timestamp() {
             functionProperties ->
                 DateTimeFunctions.unixTimeStamp(functionProperties.getQueryStartClock()),
             LONG),
-        impl(nullMissingHandling(DateTimeFunctions::unixTimeStampOf), DOUBLE, DATE),
         impl(nullMissingHandling(DateTimeFunctions::unixTimeStampOf), DOUBLE, TIMESTAMP),
+        impl(nullMissingHandling(DateTimeFunctions::unixTimeStampOf), DOUBLE, DATE),
         impl(nullMissingHandling(DateTimeFunctions::unixTimeStampOf), DOUBLE, DOUBLE));
   }
 

core/src/main/java/org/opensearch/sql/expression/function/PPLFuncImpTable.java

@@ -33,6 +33,7 @@ public class PPLFuncImpTable {
   public interface FunctionImp {
     RelDataType ANY_TYPE = TYPE_FACTORY.createSqlType(SqlTypeName.ANY);
 
+    // TODO: Support argument coercion and casting
     RexNode resolve(RexBuilder builder, RexNode... args);
 
     /**

integ-test/src/test/java/org/opensearch/sql/calcite/remote/CalciteDateTimeFunctionIT.java

@@ -5,6 +5,8 @@
 
 package org.opensearch.sql.calcite.remote;
 
+import java.io.IOException;
+import org.junit.Ignore;
 import org.opensearch.sql.ppl.DateTimeFunctionIT;
 
 public class CalciteDateTimeFunctionIT extends DateTimeFunctionIT {
@@ -14,4 +16,9 @@ public void init() throws Exception {
     enableCalcite();
     disallowCalciteFallback();
   }
+
+  // TODO: Remove this when supporting type coercion and casting with Calcite
+  @Ignore
+  @Override
+  public void testUnixTimestampWithTimestampString() throws IOException {}
 }

integ-test/src/test/java/org/opensearch/sql/ppl/DateTimeFunctionIT.java

@@ -21,6 +21,7 @@
 import org.json.JSONObject;
 import org.junit.After;
 import org.junit.Before;
+import org.junit.Ignore;
 import org.junit.jupiter.api.Test;
 import org.opensearch.sql.common.utils.StringUtils;
 
@@ -1354,6 +1355,30 @@ public void testUnixTimeStamp() throws IOException {
     verifySome(result.getJSONArray("datarows"), rows(613094400d, 1072872000d, 3404817525d));
   }
 
+  @Test
+  public void testUnixTimestampWithTimestampString() throws IOException {
+    var result =
+        executeQuery(
+            String.format(
+                "source=%s | eval f = UNIX_TIMESTAMP('1984-06-06 12:00:00.123456') | fields f",
+                TEST_INDEX_DATE));
+    verifySchema(result, schema("f", null, "double"));
+    verifySome(result.getJSONArray("datarows"), rows(4.55371200123456E8));
+  }
+
+  // TODO: Enable after fixing #3728
+  @Ignore
+  @Test
+  public void testUnixTimestampWithDateString() throws IOException {
+    var result =
+        executeQuery(
+            String.format(
+                "source=%s | eval f = UNIX_TIMESTAMP('1984-06-06 12:00:00.123456') | fields f",
+                TEST_INDEX_DATE));
+    verifySchema(result, schema("f", null, "double"));
+    verifySome(result.getJSONArray("datarows"), rows(4.55371200123456E8));
+  }
+
   @Test
   public void testPeriodAdd() throws IOException {
     var result =