trim function feature needs improvement

Description

--  the following 3 is  support:
mysql> SELECT TRIM('  bar   ');
        -> 'bar'
mysql> SELECT TRIM(BOTH 'x' FROM 'xxxbarxxx');
        -> 'bar'

mysql> SELECT TRIM(LEADING 'x' FROM 'xxxbarxxx');
        -> 'barxxx'
mysql> SELECT TRIM(TRAILING 'xyz' FROM 'barxxyz');
        -> 'barx'

Contributor guide

Tech stack

javasql

Domain

database

Issue type

feature

DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.

3

Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.

1-2 days

Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.

fresh

ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.

mostly clear

Prerequisites

JavaSQL parsing

Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.

40

Research direction

Examine the current trim function implementation in StarRocks, likely in the SQL parser and function evaluation code. Look for how similar functions like LTRIM/RTRIM are handled. The MySQL documentation specifies the syntax for LEADING, TRAILING, and BOTH with optional character removal. Focus on the parser to recognize these keywords and the execution engine to apply the specified removal. Check for any existing issues or PRs related to string functions for guidance.