This function returns a string of length n, which is string, filled from the left with expression padding.
- If the variable padding is not specified, spaces are used.
- If the string is longer than n characters, then the first n characters of the string are returned.
- Even if n is greater than 2,000,000, the result string is truncated to 2,000,000 characters.
- For filling a string from the right, please refer to function RPAD.