Function merge

datafusion/functions-aggregate/src/string_agg.rs:775–786

(
        mut acc1: Box<dyn Accumulator>,
        mut acc2: Box<dyn Accumulator>,
    )

    }

    fn merge(
        mut acc1: Box<dyn Accumulator>,
        mut acc2: Box<dyn Accumulator>,
    ) -> Result<Box<dyn Accumulator>> {
        let intermediate_state = acc2.state().and_then(|e| {
            e.iter()
                .map(|v| v.to_array())
                .collect::<Result<Vec<ArrayRef>>>()
        })?;
        acc1.merge_batch(&intermediate_state)?;
        Ok(acc1)
    }

    // ---------------------------------------------------------------
    // Tests for StringAggGroupsAccumulator