[CALCITE-6663] Support SPLIT_PART function for PostgreSQL

core/src/main/java/org/apache/calcite/runtime/SqlFunctions.java

@@ -917,8 +917,8 @@ public static List<String> split(String s, String delimiter) {
 
   /** SQL {@code SPLIT_PART(string, string, int)} function. */
   public static String splitPart(String s, String delimiter, int n) {
-    if (s == null || delimiter == null || delimiter.isEmpty() || n == 0) {
-      return null;
+    if (s == null || s.equals("") || delimiter == null || delimiter.isEmpty() || n == 0) {
+      return "";
     }
 
     String[] parts = s.split(Pattern.quote(delimiter), -1);
@@ -929,7 +929,7 @@ public static String splitPart(String s, String delimiter, int n) {
     }
 
     if (n <= 0 || n > partCount) {
-      return null;
+      return "";
     }
 
     return parts[n - 1];

core/src/test/java/org/apache/calcite/test/SqlFunctionsTest.java

@@ -1061,14 +1061,14 @@ private void checkCeil(int x, int y, int result) {
 
     assertThat(SqlFunctions.splitPart("abc,,ghi", ",", 2), is(""));
     assertThat(SqlFunctions.splitPart("", ",", 1), is(""));
-    assertThat(SqlFunctions.splitPart("abc", "", 1), is(nullValue()));
+    assertThat(SqlFunctions.splitPart("abc", "", 1), is(""));
 
-    assertThat(SqlFunctions.splitPart(null, ",", 1), is(nullValue()));
-    assertThat(SqlFunctions.splitPart("abc,def", null, 1), is(nullValue()));
-    assertThat(SqlFunctions.splitPart("abc,def", ",", 0), is(nullValue()));
+    assertThat(SqlFunctions.splitPart(null, ",", 1), is(""));
+    assertThat(SqlFunctions.splitPart("abc,def", null, 1), is(""));
+    assertThat(SqlFunctions.splitPart("abc,def", ",", 0), is(""));
 
-    assertThat(SqlFunctions.splitPart("abc,def", ",", 3), is(nullValue()));
-    assertThat(SqlFunctions.splitPart("abc,def", ",", -3), is(nullValue()));
+    assertThat(SqlFunctions.splitPart("abc,def", ",", 3), is(""));
+    assertThat(SqlFunctions.splitPart("abc,def", ",", -3), is(""));
   }
 
   @Test void testByteString() {

site/_docs/reference.md

@@ -2927,6 +2927,7 @@ In the following:
 | s | SOUNDEX(string)                                | Returns the phonetic representation of *string*; return original *string* if *string* is encoded with multi-byte encoding such as UTF-8
 | m s | SPACE(integer)                               | Returns a string of *integer* spaces; returns an empty string if *integer* is less than 1
 | b | SPLIT(string [, delimiter ])                   | Returns the string array of *string* split at *delimiter* (if omitted, default is comma).  If the *string* is empty it returns an empty array, otherwise, if the *delimiter* is empty, it returns an array containing the original *string*.
+| p | SPLIT_PART(string, delimiter, position)        | Returns the *position* number field in *string* using *delimiter*; returns empty string if *position* is less than 1 or greater than the number of fields, and the position can be negative to count from the end.
 | f s | STARTSWITH(string1, string2)                 | Returns whether *string2* is a prefix of *string1*
 | b p | STARTS_WITH(string1, string2)                | Equivalent to `STARTSWITH(string1, string2)`
 | m | STRCMP(string, string)                         | Returns 0 if both of the strings are same and returns -1 when the first argument is smaller than the second and 1 when the second one is smaller than the first one