Yikun commented on code in PR #36464: URL: https://github.com/apache/spark/pull/36464#discussion_r873426625
########## python/pyspark/pandas/groupby.py: ########## @@ -2110,22 +2110,79 @@ def _limit(self, n: int, asc: bool) -> FrameLike: groupkey_scols = [psdf._internal.spark_column_for(label) for label in groupkey_labels] sdf = psdf._internal.spark_frame - tmp_col = verify_temp_column_name(sdf, "__row_number__") + tmp_row_num_col = verify_temp_column_name(sdf, "__row_number__") + window = Window.partitionBy(*groupkey_scols) # This part is handled differently depending on whether it is a tail or a head. - window = ( - Window.partitionBy(*groupkey_scols).orderBy(F.col(NATURAL_ORDER_COLUMN_NAME).asc()) + ordered_window = ( + window.orderBy(F.col(NATURAL_ORDER_COLUMN_NAME).asc()) if asc - else Window.partitionBy(*groupkey_scols).orderBy( - F.col(NATURAL_ORDER_COLUMN_NAME).desc() - ) + else window.orderBy(F.col(NATURAL_ORDER_COLUMN_NAME).desc()) ) - sdf = ( - sdf.withColumn(tmp_col, F.row_number().over(window)) - .filter(F.col(tmp_col) <= n) - .drop(tmp_col) - ) + if n >= 0 or LooseVersion(pd.__version__) < LooseVersion("1.4.0"): + + sdf = ( + sdf.withColumn(tmp_row_num_col, F.row_number().over(ordered_window)) + .filter(F.col(tmp_row_num_col) <= n) + .drop(tmp_row_num_col) + ) + else: + # Pandas supports Groupby positional indexing since v1.4.0 + # https://pandas.pydata.org/docs/whatsnew/v1.4.0.html#groupby-positional-indexing + # + # To support groupby positional indexing, we need add two columns to help we filter + # target rows: + # - Add `__row_number__` and `__group_count__` columns. + # - Use `F.col(tmp_row_num_col) - F.col(tmp_cnt_col) <= positional_index_number` to + # filter target rows. + # - Then drop `__row_number__` and `__group_count__` columns. + # + # For example for the dataframe: + # >>> df = ps.DataFrame([["g", "g0"], + # ... ["g", "g1"], + # ... ["g", "g2"], + # ... ["g", "g3"], + # ... ["h", "h0"], + # ... ["h", "h1"]], columns=["A", "B"]) + # >>> df.groupby("A").head(-1) + # + # Below is an example to show the `__row_number__` column and `__group_count__` column + # for above df: + # >>> sdf.withColumn(tmp_row_num_col, F.row_number().over(window)) + # .withColumn(tmp_cnt_col, F.count("*").over(window)).show() + # +---------------+------------+---+---+------------+--------------+---------------+ + # |__index_level..|__groupkey..| A| B|__natural_..|__row_number__|__group_count__| + # +---------------+------------+---+---+------------+--------------+---------------+ + # | 0| g| g| g0| 17179869184| 1| 4| + # | 1| g| g| g1| 42949672960| 2| 4| + # | 2| g| g| g2| 60129542144| 3| 4| + # | 3| g| g| g3| 85899345920| 4| 4| + # | 4| h| h| h0|111669149696| 1| 2| + # | 5| h| h| h1|128849018880| 2| 2| + # +---------------+------------+---+---+------------+--------------+---------------+ + # + # The limit n is `-1`, we need to filter rows[:-1] in each group: + # + # >>> sdf.withColumn(tmp_row_num_col, F.row_number().over(window)) + # .withColumn(tmp_cnt_col, F.count("*").over(window)) + # .filter(F.col(tmp_row_num_col) - F.col(tmp_cnt_col) <= -1).show() Review Comment: @zhengruifeng I think `lag` is better in here: 1 `WindowsExec` + 2 `sort` + 1 `shuffle`, same cost with orignal implments <details><summary>== Physical Plan ==</summary> ``` == Physical Plan == AdaptiveSparkPlan isFinalPlan=false +- Project [__index_level_0__#0, __groupkey_0__#19L, a#1L, b#2L, c#3L, __natural_order__#8L] +- Filter isnull(__tmp_lag__#447) +- Window [lag(0, -2, null) windowspecdefinition(__groupkey_0__#19L, __natural_order__#8L ASC NULLS FIRST, specifiedwindowframe(RowFrame, -2, -2)) AS __tmp_lag__#447], [__groupkey_0__#19L], [__natural_order__#8L ASC NULLS FIRST] +- Sort [__groupkey_0__#19L ASC NULLS FIRST, __natural_order__#8L ASC NULLS FIRST], false, 0 +- Exchange hashpartitioning(__groupkey_0__#19L, 200), ENSURE_REQUIREMENTS, [id=#1049] +- Project [__index_level_0__#0, a#1L AS __groupkey_0__#19L, a#1L, b#2L, c#3L, __natural_order__#8L] +- Project [__index_level_0__#0, a#1L, b#2L, c#3L, monotonically_increasing_id() AS __natural_order__#8L] +- Scan ExistingRDD arrow[__index_level_0__#0,a#1L,b#2L,c#3L] ``` </details> So, I updated it in my PR. -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. To unsubscribe, e-mail: reviews-unsubscr...@spark.apache.org For queries about this service, please contact Infrastructure at: us...@infra.apache.org --------------------------------------------------------------------- To unsubscribe, e-mail: reviews-unsubscr...@spark.apache.org For additional commands, e-mail: reviews-h...@spark.apache.org