Extend splitByString for empty separators

dbms/src/Functions/FunctionsStringArray.h

@@ -214,9 +214,6 @@ class SplitByStringImpl
                 ErrorCodes::ILLEGAL_COLUMN);
 
         sep = col->getValue<String>();
-
-        if (sep.empty())
-            throw Exception("Illegal separator for function " + getName() + ". Must be not empty.", ErrorCodes::BAD_ARGUMENTS);
     }
 
     /// Returns the position of the argument that is the column of strings
@@ -239,15 +236,27 @@ class SplitByStringImpl
             return false;
 
         token_begin = pos;
-        pos = reinterpret_cast<Pos>(memmem(pos, end - pos, sep.data(), sep.size()));
 
-        if (pos)
+        if (sep.empty())
         {
+            pos += 1;
             token_end = pos;
-            pos += sep.size();
+
+            if (pos == end)
+                pos = nullptr;
         }
         else
-            token_end = end;
+        {
+            pos = reinterpret_cast<Pos>(memmem(pos, end - pos, sep.data(), sep.size()));
+
+            if (pos)
+            {
+                token_end = pos;
+                pos += sep.size();
+            }
+            else
+                token_end = end;
+        }
 
         return true;
     }

dbms/tests/queries/0_stateless/01100_split_by_string.reference

@@ -0,0 +1,7 @@
+['cde','cde']
+['','cde','cde','']
+['','','','']
+['','']
+['a','b','c','d','e']
+['hello','world']
+['gbye','bug']

dbms/tests/queries/0_stateless/01100_split_by_string.sql

@@ -0,0 +1,6 @@
+select splitByString('ab', 'cdeabcde');
+select splitByString('ab', 'abcdeabcdeab');
+select splitByString('ab', 'ababab');
+select splitByString('ababab', 'ababab');
+select splitByString('', 'abcde');
+select splitByString(', ', x) from (select arrayJoin(['hello, world', 'gbye, bug']) x);

docs/en/query_language/functions/splitting_merging_functions.md

@@ -5,9 +5,40 @@
 Splits a string into substrings separated by 'separator'.'separator' must be a string constant consisting of exactly one character.
 Returns an array of selected substrings. Empty substrings may be selected if the separator occurs at the beginning or end of the string, or if there are multiple consecutive separators.
 
+**Example:**
+
+```sql
+SELECT splitByChar(',', '1,2,3,abcde')
+```
+```text
+┌─splitByChar(',', '1,2,3,abcde')─┐
+│ ['1','2','3','abcde']           │
+└─────────────────────────────────┘
+```
+
 ## splitByString(separator, s)
 
-The same as above, but it uses a string of multiple characters as the separator. The string must be non-empty.
+The same as above, but it uses a string of multiple characters as the separator. If the string is empty, it will split the string into an array of single characters.
+
+**Example:**
+
+```sql
+SELECT splitByString(', ', '1, 2 3, 4,5, abcde')
+```
+```text
+┌─splitByString(', ', '1, 2 3, 4,5, abcde')─┐
+│ ['1','2 3','4,5','abcde']                 │
+└───────────────────────────────────────────┘
+```
+
+```sql
+SELECT splitByString('', 'abcde')
+```
+```text
+┌─splitByString('', 'abcde')─┐
+│ ['a','b','c','d','e']      │
+└────────────────────────────┘
+```
 
 ## arrayStringConcat(arr\[, separator\])