PostgreSQL成本因子校準（二）

時間 2019-12-01

標籤 postgresql 成本因子校準欄目 Postgre SQL 简体版

原文原文鏈接

校準參數：sql

　　random_page_cost(stap), cpu_index_cost（公式）, cpu_operator_cost（公式）數據庫

步驟：緩存

1.爲了方便計算，其它參數常量設爲1：dom

　　set random_page_cost=1;ide

　　set cpu_tuple_cost=1;oop

　　set cpu_index_tuple_cost=1;post

　　set cpu_operator_cost=1;spa

　　set enable_seqscan=off; set enable_bitmapscan=off;操作系統

　　explain (analyze,verbose,costs,buffers,timing) select * from tbl_cost_align where id>1998999963;code

                                                                          QUERY PLAN                              
                                            
------------------------------------------------------------------------------------------------------------------
--------------------------------------------
 Index Scan using tbl_cost_align_id_idx on public.tbl_cost_align  (cost=174.00..20870.00 rows=5205 width=45) (actu
al time=0.076..32827.810 rows=4908 loops=1)
   Output: id, info, crt_time
   Index Cond: (tbl_cost_align.id > 1998999963)
   Buffers: shared hit=113 read=4811
 Planning time: 9.621 ms
 Execution time: 32835.450 ms
(6 rows)

Explain輸出

分析：執行計劃代表這是個索引掃描, 至於掃了多少個數據塊是未知的, 索引的tuples也是未知的, 已知的是cost和rows

20870.00 = blocks*random_page_cost + cpu_tuple_cost*5205+ cpu_index_tuple_cost*? + cpu_operator_cost(1)*?

2.更改cpu_operator_cost

　　set cpu_operator_cost=2;

　　set enable_seqscan=off; set enable_bitmapscan=off; explain (analyze,verbose,costs,buffers,timing) select * from tbl_cost_align where id>1998999963;

                                                                        QUERY PLAN                                
                                         
------------------------------------------------------------------------------------------------------------------
-----------------------------------------
 Index Scan using tbl_cost_align_id_idx on public.tbl_cost_align  (cost=348.00..26249.00 rows=5205 width=45) (actu
al time=0.032..13.791 rows=4908 loops=1)
   Output: id, info, crt_time
   Index Cond: (tbl_cost_align.id > 1998999963)
   Buffers: shared hit=4924
 Planning time: 0.165 ms
 Execution time: 20.082 ms
(6 rows)

Explain輸出

分析：26249.00 = blocks*random_page_cost + cpu_tuple_cost*5205+ cpu_index_tuple_cost*? + cpu_operator_cost(2)*?，獲得本例經過索引掃描的條數：26249.00-20870.00=5379

等式變爲20870.00 = blocks*random_page_cost + cpu_tuple_cost*5205+ cpu_index_tuple_cost*5205 + cpu_operator_cost(1)*5379

3.一樣的方法求blocks

　　set random_page_cost = 2;

set enable_seqscan=off; set enable_bitmapscan=off; explain (analyze,verbose,costs,buffers,timing) select * from tbl_cost_align where id>1998999963;

                                                                        QUERY PLAN                                
                                         
------------------------------------------------------------------------------------------------------------------
-----------------------------------------
 Index Scan using tbl_cost_align_id_idx on public.tbl_cost_align  (cost=348.00..31330.00 rows=5205 width=45) (actu
al time=0.032..13.154 rows=4908 loops=1)
   Output: id, info, crt_time
   Index Cond: (tbl_cost_align.id > 1998999963)
   Buffers: shared hit=4924
 Planning time: 0.165 ms
 Execution time: 19.309 ms
(6 rows)

Explain

blocks=31330.00-26249.00=5081.00

更新公式：20870.00 = 5081*random_page_cost + cpu_tuple_cost*5205+ cpu_index_tuple_cost*5205 + cpu_operator_cost*5379

4.經過stap統計random_page_cost

　　CHECKPOINT關閉數據庫；

　　將操做系統的緩存刷入硬盤 sync; echo 3 > /proc/sys/vm/drop_caches

　　以非0號cpu親和啓動數據庫 taskset -c 1 /usr/local/pgsql/bin/postgres >/dev/null 2>&1

　　鏈接數據庫，查看pid：select pg_backend_pid();

　　啓動監控代碼：同上

　　執行：set enable_seqscan=off; set enable_bitmapscan=off; explain (analyze,verbose,costs,buffers,timing) select * from tbl_cost_align where id>1998999963;

　　查看stap及explain輸出：

13094**4854**8752355
query__done explain (analyze,verbose,costs,buffers,timing) select * from tbl_cost_align where
id>1998999963;pid:13094
     value |-------------------------------------------------- count
      2048 |                                                      0
      4096 |                                                      0
      8192 |                                                      6
     16384 |                                                     27
     32768 |                                                      7
     65536 |                                                      3
    131072 |                                                      0
    262144 |                                                     21
    524288 |                                                      2
   1048576 |                                                      9
   2097152 |@@@@@@@@@                                           486
   4194304 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@                1782
   8388608 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@  2495
  16777216 |                                                      8
  33554432 |                                                      4
  67108864 |                                                      2
 134217728 |                                                      1
 268435456 |                                                      1
 536870912 |                                                      0
1073741824 |                                                      0

stap輸出

                                                                         QUERY PLAN                               
                                          
------------------------------------------------------------------------------------------------------------------
------------------------------------------
 Index Scan using tbl_cost_align_id_idx on public.tbl_cost_align  (cost=0.43..20415.52 rows=5205 width=45) (actual
 time=6.920..42273.537 rows=4908 loops=1)
   Output: id, info, crt_time
   Index Cond: (tbl_cost_align.id > 1998999963)
   Buffers: shared hit=110 read=4814
 Planning time: 148.140 ms
 Execution time: 42282.361 ms
(6 rows)