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https://issues.apache.org/jira/browse/FLINK-36377?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17888569#comment-17888569
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xuyang commented on FLINK-36377:
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This feature makes sense to me. BTW, do other complex types (such as Raw, Map,
Array, etc.) have similar issues?
> Support the use of the LAST_VALUE aggregate function on ROW type data
> ----------------------------------------------------------------------
>
> Key: FLINK-36377
> URL: https://issues.apache.org/jira/browse/FLINK-36377
> Project: Flink
> Issue Type: Improvement
> Components: Runtime / State Backends
> Reporter: Yang Li
> Priority: Major
>
> h2. Introduction
> In Flink, after applying a group by, users may use LAST_VALUE to process
> certain fields to ensure that all fields have corresponding aggregation
> functions. Currently, LAST_VALUE does not support the ROW type syntax, so
> users always apply the LAST_VALUE function to each individual field
> separately, as shown below.
> SELECT
> LAST_VALUE(bool_a) AS last_bool_a,
> LAST_VALUE(int2_b) AS last_int2_b,
> LAST_VALUE(int4_c) AS last_int4_c,
> LAST_VALUE(int8_d) AS last_int8_d,
> LAST_VALUE(float4_e) AS last_float4_e,
> LAST_VALUE(float4_f) AS last_float4_f,
> LAST_VALUE(numeric_g) AS last_numeric_g,
> LAST_VALUE(text_m) AS last_text_m,
> LAST_VALUE(varchar_p) AS last_varchar_p,
> date_h
> FROM source_table
> GROUP BY date_h
>
> If the upstream operator is a retract stream, this approach will lead to
> redundant StateMap traversal. To facilitate retraction, Flink's internal{{
> LastValueWithRetractAggFunction}} will store all historical data related to
> the primary key. When the last value is deleted, it will traverse all keys in
> the{{ orderToValue}} (which maps timestamps to data) and this {{MapView}} is
> stored in the form of {{{}StateMap{}}}. More {{LAST_VALUE}} functions leads
> to more times the read and write operations of RocksDB. Therefore, I advocate
> for handling {{ROW}} types with {{{}LAST_VALUE{}}}, allowing support for all
> fields with just one {{LAST_VALUE}} function as below.
> SELECT
> LAST_VALUE(
> ROW(
> bool_a,
> int2_b,
> int4_c,
> int8_d,
> float4_e,
> float4_f,
> numeric_g,
> text_m,
> varchar_p
> )
> ) AS row_data,
> date_h
> FROM source_table
> GROUP BY date_h
> The experiment indicates that applying the {{ROW}} type to the {{LAST_VALUE}}
> function can improve the processing speed for retract streams, but has no
> effect on append-only streams.
> h2. Evaluation：
> The throughput of jobs was compared based on whether the {{ROW}} type was
> used in the {{LAST_VALUE}} function, considering both retract and append-only
> scenarios.
> h3. Retraction
> Use a deduplication operator to convert the append-only stream generated by
> datagen into a retract stream.
> Two jobs show little difference in throughput (Row 4817: Mean 1808).
> Through flame graph analysis, applying the ROW type to the LAST_VALUE
> function reduces the consumption of the aggregate function calls to
> accumulate,
> with CPU usage for accumulate being (ROW 20.02%: Separated 66.98%).
> LastValueWithRetractAccumulator uses MapState storage MapView.
> Therefore, updating the LastValueWithRetractAccumulator requires reading from
> or writing to RocksDB.
> h3. AppendOnly
> Two jobs show little difference in throughput (Row 13411: Mean 10673).
> Further examination of the flame graphs for both processes reveals that the
> bottleneck
> for both jobs lies in getting {{RocksDBValueState}} which is called by
> {{{}GroupFunction{}}}.
> Using {{ROW}} aggregation does not yield significant optimization in this
> part. I suspect it's
> because Flink uses RowData to store data from multiple Accumulators, and
> every time
> the {{accState}} invokes the {{value}} method, it reads all the Accumulators
> at the same time.
> Therefore, the use of ROW optimization might not be very effective.
> h2. Conclusion
> # Using ROW type for LAST_VALUE Aggregation can improve the processing speed
> for retract streams, with effectiveness proportional to the number of fields
> contained in the {{{}ROW{}}}.
> # Using ROW type for LAST_VALUE Aggregation results in limited improvements
> , as the optimization effect on state backend read speed is not significant.
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