class DuckDB::PreparedStatement

def bind_decimal(index, value)

stmt.bind_decimal(1, BigDecimal('987654.321'))
stmt = PreparedStatement.new(con, sql)
sql ='SELECT value FROM decimals WHERE decimal = ?'
con = db.connect
db = DuckDB::Database.open('duckdb_database')
require 'duckdb'

that can be parsed into a BigDecimal.
The second argument value is to expected BigDecimal value or any value
The index of first parameter is 1 not 0.
The first argument is index of parameter.
binds i-th parameter with SQL prepared statement. 
def bind_decimal(index, value)
  decimal = _parse_deciaml(value)
  lower, upper = decimal_to_hugeint(decimal)
  width = decimal.to_s('F').gsub(/[^0-9]/, '').length
  _bind_decimal(index, lower, upper, width, decimal.scale)
end