Create 20240906_02.md

Author: digoal

202409/20240906_02.md

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+## PostgreSQL 18 preview - 窗口查询性能提升40%(100万个窗口时)     
+                                                                                    
+### 作者                                                        
+digoal                                                        
+                                                               
+### 日期                                                             
+2024-09-06                                                       
+                                                            
+### 标签                                                          
+PostgreSQL , PolarDB , DuckDB , 窗口查询 , 性能 , 窗口数       
+                                                                                   
+----                                                            
+                                                                          
+## 背景       
+PostgreSQL 18窗口查询性能提升优化, 100万个窗口时性能提升40%, 窗口越多性能提升越明显.  代码优化参考如下说明, 看起来就是把tuplestore_clear的返回指针回传了, 减少了重复调用.   
+  
+类似 `update/delete returning` 减少重复查询?    
+  
+![pic](20240906_02_pic_001.png)    
+  
+https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=908a968612f9ed61911d8ca0a185b262b82f1269  
+```  
+Optimize WindowAgg's use of tuplestores  
+author	David Rowley <[email protected]>	  
+Thu, 5 Sep 2024 04:18:30 +0000 (16:18 +1200)  
+committer	David Rowley <[email protected]>	  
+Thu, 5 Sep 2024 04:18:30 +0000 (16:18 +1200)  
+commit	908a968612f9ed61911d8ca0a185b262b82f1269  
+tree	bf1957ccad3aab816fc5b0175c2294c099dc6680	tree  
+parent	19b861f880166fbdb67d268955e590881376f876	commit | diff  
+  
+Optimize WindowAgg's use of tuplestores  
+  
+When WindowAgg finished one partition of a PARTITION BY, it previously  
+would call tuplestore_end() to purge all the stored tuples before again  
+calling tuplestore_begin_heap() and carefully setting up all of the  
+tuplestore read pointers exactly as required for the given frameOptions.  
+Since the frameOptions don't change between partitions, this part does  
+not make much sense.  For queries that had very few rows per partition,  
+the overhead of this was very large.  
+  
+It seems much better to create the tuplestore and the read pointers once  
+and simply call tuplestore_clear() at the end of each partition.  
+tuplestore_clear() moves all of the read pointers back to the start  
+position and deletes all the previously stored tuples.  
+  
+A simple test query with 1 million partitions and 1 tuple per partition  
+has been shown to run around 40% faster than without this change.  The  
+additional effort seems to have mostly been spent in malloc/free.  
+  
+Making this work required adding a new bool field to WindowAggState  
+which had the unfortunate effect of being the 9th bool field in a group  
+resulting in the struct being enlarged.  Here we shuffle the fields  
+around a little so that the two bool fields for runcondition relating  
+stuff fit into existing padding.  Also, move the "runcondition" field to  
+be near those.  This frees up enough space with the other bool fields so  
+that the newly added one fits into the padding bytes.  This was done to  
+address a very small but apparent performance regression with queries  
+containing a large number of rows per partition.  
+  
+Reviewed-by: Ashutosh Bapat <[email protected]>  
+Reviewed-by: Tatsuo Ishii <[email protected]>  
+Discussion: https://postgr.es/m/CAHoyFK9n-QCXKTUWT_xxtXninSMEv%2BgbJN66-y6prM3f4WkEHw%40mail.gmail.com  
+```  
+