Optimization of lazyagg query rewriting in parsing data warehouse

Abstract ： This paper deals with Lazy Agg Query rewrite optimization and GaussDB(DWS) Provided Lazy Agg Rewrite rules .

This article is shared from Huawei cloud community 《GaussDB(DWS) lazyagg Query rewrite optimization resolution 【 Gauss is not a mathematician this time 】》, author ： OreoreO .

The aggregation operation groups the query results by the values of one or more columns , A set of equal values . Aggregation is a common operation and is widely used in financial customers . For example, the following statement ：

SELECT a, count(a) FROM t1 GROUP BY a; --  Press a Group and calculate the number of duplicate values in the group 

One 、Lazy Agg Rewriting rule

In the case of large amount of data , Due to the large amount of data, the footwall , The execution time of aggregation operation becomes a performance bottleneck , As a result, the whole query execution efficiency is very poor . for example ：

SELECT t2.b, sum(cc) FROM (SELECT b, sum(c) AS cc FROM t1 GROUP BY b) AS s, t2 WHERE s.b=t2.b GROUP BY t2.b;

Subquery pair t1.b Columns are aggregated , Yes t1.c Summation , In an external query , There are also aggregation operations , Aggregate sum columns for subqueries cc Summation . For such statements , When the aggregation operation of sub query is time-consuming , Query rewriting rules can be used to eliminate the aggregation of subqueries , The aggregation function of the external query uniformly completes the aggregation operation . Eliminating a subquery may result in an increase in the number of rows in the subquery , But for the sub query aggregation operation t1.b Column distinct Scenarios with high values , The number of rows after the sub query aggregation operation will not be significantly reduced compared with the original table , Will not cause the outer layer JOIN A large increase in the amount of computation . That is, the statement can be rewritten as ：

SELECT t2.b, sum(cc) FROM (SELECT b, c AS cc FROM t1) AS s, t2 WHERE s.b=t2.b GROUP BY t2.b;

This rewrite rule is called Lazy Agg, It is applicable to the large amount of base table data distinct Scenarios with high values . If there are fewer duplicate values , Then eliminating the aggregation operation will lead to Join After that, the number of lines surged ,Join Poor performance , Therefore, it is necessary to Agg Push down to Join Before , Through advance Agg Reduced operation Join The number of rows of the result , This rewrite rule is called Eager Agg.

Two 、GaussDB(DWS) lazyagg Optimize

To make tuning less difficult , Improve product ease of use ,GaussDB(DWS) Provides lazyagg Query rewrite optimization rules , Can be set by guc Parameters rewrite_rule contain ’lazyagg’ Use Lazy Agg Query rewrite optimization . Turn on lazyagg After query rewrite optimization , For the scenario that meets the conditions, the aggregation operation in the sub query will be optimized and eliminated . The original plan is as follows ：

lazyagg Rewrite the optimized plan as follows ：

You can see that compared with the original plan ,lazyagg After rewriting the optimization, the aggregation operation in the original plan is eliminated , namely 7 Number Subquery Scan Operator and 8 Number HashAggregate operator .

3、 ... and 、lazyagg Optimize specifications

1. The sub query can be a single aggregate query or a query containing aggregate sub set operations . Collection operations only support UNION ALL, Some branch sub queries can be aggregated and eliminated . Subquery must be JOIN One of the tables （ be not in TargetList、Where Clause, etc ）.
2. Support all external queries Agg The parameter column is contained in the... Of one of its subqueries Agg Function column , The aggregation operation of the sub query can be eliminated .
3. Support all kinds of aggregation functions with correct results after eliminating the aggregation operation of sub queries . See the following table for the correctness of aggregation function type results ：

4. Scene constraint
On the basis of the above scenario expansion , For scenarios that may lead to incorrect results , No query rewriting , Including but not limited to ：

• Eliminating is not supported Agg Function type .
• The subquery contains other conditions or operators , Will result in error after rewriting , for example HAVING、window agg、LIMIT、OFFSET、AP function、distinct、recursive etc. .
• Outer layer Agg Parameter column 、GROUP BY Column or JOIN Column contains volatile function , Such as random、timeofday etc. .
• Subquery Agg Out of function 、 External query Agg There are other expressions or function operations in the function , Such as sub query Agg Function column is sum+1、max+max(d), External query Agg Function column is sum(cc+1) etc. .
• For external queries JOIN Column 、GROUP BY Columns or other conditions contain subqueries Agg Function column .
• Subquery in LEFT JOIN、RIGHT JOIN Of inner Edge or FULL JOIN in , And subquery Agg Function is count, External query Agg Function is sum Of .

Four 、 Conclusion

Through the analysis of this paper , I believe the user friends have fully understood Lazy Agg Rewrite optimized usage scenarios , as well as GaussDB(DWS) Of lazyagg Realization way . I hope that the majority of users can have an in-depth understanding of , Yes GaussDB(DWS) Have a strong interest in and deeply participate in the performance tuning of .

Reference documents ：
GaussDB(DWS) Performance Tuning Series 4 ： One of the eighteen martial arts SQL rewrite

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