Reuse ScalarFunc::find_str in ScalarFunc::instr (#4446)
Reuse ScalarFunc::find_str in ScalarFunc::instr (#4446)

Signed-off-by: iosmanthus <baa76a6a5829eecb15ba169cea72c8e906bb18e1@gmail.com>
pingcap/tikv,BusyJay/tikv,BusyJay/tikv,BusyJay/tikv,c4pt0r/tikv,pingcap/tikv,BusyJay/tikv