postgresql查詢規劃過程當中，查詢請求的不一樣執行方案是經過創建不一樣的路徑來表達的，在生成許多符合條件的路徑以後，要從中選擇出代價最小的路徑，把它轉化爲一個計劃，傳遞給執行器執行，規劃器的核心工做就是生成多條路徑，而後從中找出最優的那一條。html
評估路徑優劣的依據是用系統表pg_statistic中的統計信息估算出來的不一樣路徑的代價(cost)，PostgreSQL估計計劃成本的方式：基於統計信息估計計劃中各個節點的成本。PostgreSQL會分析各個表來獲取一個統計信息樣本（這個操做一般是由autovacuum這個守護進程週期性的執行analyze，來收集這些統計信息，而後保存到pg_statistic和pg_class裏面）。git
# - Planner Cost Constants -
#seq_page_cost = 1.0 # measured on an arbitrary scale 順序磁盤掃描時單個頁面的開銷 #random_page_cost = 4.0 # same scale as above 隨機磁盤訪問時單頁面的讀取開銷 #cpu_tuple_cost = 0.01 # same scale as above cpu處理每一行的開銷 #cpu_index_tuple_cost = 0.005 # same scale as above cpu處理每一個索引行的開銷 #cpu_operator_cost = 0.0025 # same scale as above cpu處理每一個運算符或者函數調用的開銷 #parallel_tuple_cost = 0.1 # same scale as above 計算並行處理的成本，若是成本高於非並行，則不會開啓並行處理。 #parallel_setup_cost = 1000.0 # same scale as above #min_parallel_relation_size = 8MB #effective_cache_size = 4GB 再一次索引掃描中可用的文件系統內核緩衝區有效大小 也可使用 show all的方式查看
--查看錶信息
db_jcxxglpt=# \d t_jcxxgl_tjaj
Table "db_jcxx.t_jcxxgl_tjaj" Column | Type | Modifiers --------------+--------------------------------+----------- c_bh | character(32) | not null c_xzdm | character varying(300) | c_jgid | character(32) | c_ajbm | character(22) | ... Indexes: "t_jcxxgl_tjaj_pkey" PRIMARY KEY, btree (c_bh) "idx_ttjaj_cah" btree (c_ah) "idx_ttjaj_dslrq" btree (d_slrq) 首先更新統計信息vacuum analyze t_jcxxgl_tjaj，許多時候可能由於統計信息的不許確致使了不正常的執行計劃 --執行計劃，全表掃描 db_jcxxglpt=# explain (analyze,verbose,costs,buffers,timing)select c_bh,c_xzdm,c_jgid,c_ajbm from t_jcxxgl_tjaj where d_slrq >='2018-03-18'; QUERY PLAN ------------------------------------------------------------------------------------------------------------ Seq Scan on db_jcxx.t_jcxxgl_tjaj (cost=0.00..9.76 rows=3 width=96) (actual time=1.031..1.055 rows=3 loops =1) Output: c_bh, c_xzdm, c_jgid, c_ajbm Filter: (t_jcxxgl_tjaj.d_slrq >= '2018-03-18'::date) Rows Removed by Filter: 138 Buffers: shared hit=8 Planning time: 6.579 ms Execution time: 1.163 ms (7 rows) --執行計劃，關閉全表掃描 db_jcxxglpt=# set session enable_seqscan = off; SET db_jcxxglpt=# explain (analyze,verbose,costs,buffers,timing)select c_bh,c_xzdm,c_jgid,c_ajbm from t_jcxxgl_tjaj where d_slrq >='2018-03-18'; QUERY PLAN ------------------------------------------------------------------------------------------------------------ Index Scan using idx_ttjaj_dslrq on db_jcxx.t_jcxxgl_tjaj (cost=0.14..13.90 rows=3 width=96) (actual time=0.012..0.026 rows=3 loops=1) Output: c_bh, c_xzdm, c_jgid, c_ajbm Index Cond: (t_jcxxgl_tjaj.d_slrq >= '2018-03-18'::date) Buffers: shared hit=4 Planning time: 0.309 ms Execution time: 0.063 ms (6 rows) d_slrq上面有btree索引，可是查看執行計劃並無走索引，這是爲何呢？ 代價計算： 一個路徑的估算由三部分組成：啓動代價(startup cost)，總代價(totalcost)，執行結果的排序方式(pathkeys) 代價估算公式：總代價=啓動代價+I/O代價+CPU代價（cost=S+P+W*T） P:執行時要訪問的頁面數，反應磁盤的I/O次數 T:表示在執行時所要訪問的元組數，反映了cpu開銷 W:表示磁盤I/O代價和CPU開銷建的權重因子 統計信息：統計信息的其中一部分是每一個表和索引中項的總數，以及每一個表和索引佔用的磁盤塊數。這些信息保存在pg_class表的reltuples和relpages列中。咱們能夠這樣查詢相關信息： --查看統計信息 db_jcxxglpt=# select relpages,reltuples from pg_class where relname ='t_jcxxgl_tjaj'; relpages | reltuples ----------+----------- 8 | 141 (1 row) total_cost = 1(seq_page_cost)*8(磁盤總頁數)+0.01(cpu_tuple_cost)*141(表的總記錄數)+0.0025(cpu_operation_cost)*141(表的總記錄數)=9.7625 能夠看到走索引的cost=13.90比全表掃描cost=9.76要大。在表較小的狀況下，全表掃描比索引掃描更有效， index scan 至少要發生兩次I/O，一次是讀取索引塊，一次是讀取數據塊。
/*
* cost_seqscan * Determines and returns the cost of scanning a relation sequentially. * * 'baserel' is the relation to be scanned * 'param_info' is the ParamPathInfo if this is a parameterized path, else NULL */ void cost_seqscan(Path *path, PlannerInfo *root, RelOptInfo *baserel, ParamPathInfo *param_info) { Cost startup_cost = 0; Cost cpu_run_cost; Cost disk_run_cost; double spc_seq_page_cost; QualCost qpqual_cost; Cost cpu_per_tuple; /* Should only be applied to base relations */ Assert(baserel->relid > 0); Assert(baserel->rtekind == RTE_RELATION); /* Mark the path with the correct row estimate */ if (param_info) path->rows = param_info->ppi_rows; else path->rows = baserel->rows; if (!enable_seqscan) startup_cost += disable_cost; /* fetch estimated page cost for tablespace containing table */ get_tablespace_page_costs(baserel->reltablespace, NULL,&spc_seq_page_cost); /* * disk costs */ disk_run_cost = spc_seq_page_cost * baserel->pages; /* CPU costs */ get_restriction_qual_cost(root, baserel, param_info, &qpqual_cost); startup_cost += qpqual_cost.startup; cpu_per_tuple = cpu_tuple_cost + qpqual_cost.per_tuple; cpu_run_cost = cpu_per_tuple * baserel->tuples; /* tlist eval costs are paid per output row, not per tuple scanned */ startup_cost += path->pathtarget->cost.startup; cpu_run_cost += path->pathtarget->cost.per_tuple * path->rows; /* Adjust costing for parallelism, if used. */ if (path->parallel_workers > 0) { double parallel_divisor = get_parallel_divisor(path); /* The CPU cost is divided among all the workers. */ cpu_run_cost /= parallel_divisor; /* * It may be possible to amortize some of the I/O cost, but probably * not very much, because most operating systems already do aggressive * prefetching. For now, we assume that the disk run cost can't be * amortized at all. */ /* * In the case of a parallel plan, the row count needs to represent * the number of tuples processed per worker. */ path->rows = clamp_row_est(path->rows / parallel_divisor); } path->startup_cost = startup_cost; path->total_cost = startup_cost + cpu_run_cost + disk_run_cost; }
慢SQL：
select c_ajbh, c_ah, c_cbfy, c_cbrxm, d_larq, d_jarq, n_dbjg, c_yqly from db_zxzhld.t_zhld_db dbxx join db_zxzhld.t_zhld_ajdbxx dbaj on dbxx.c_bh = dbaj.c_dbbh where dbxx.n_valid=1 and dbxx.n_state in (1,2,3) and dbxx.c_dbztbh='1003' and dbaj.c_zblx='1003' and dbaj.c_dbfy='0' and dbaj.c_gy = '2550' and c_ajbh in (select distinct c_ajbh from db_zxzhld.t_zhld_zbajxx where n_dbzt = 1 and c_zblx = '1003' and c_gy = '2550' ) order by d_larq asc, c_ajbh asc limit 15 offset 0 慢sql耗時：7s 咋們先過下這個sql是幹什麼的、首先dbxx和dbaj的一個join鏈接而後dbaj.c_ajbh要包含在zbaj表裏面，作了個排序，取了15條記錄、大概就這樣。 Sql有個缺點就是我不知道查詢的字段是從那個表裏面取的、建議加上表別名.字段。 查看該sql的表的數據量： db_zxzhld.t_zhld_db ：1311 db_zxzhld.t_zhld_ajdbxx ：341296 db_zxzhld.t_zhld_zbajxx ：1027619 執行計劃： 01 Limit (cost=36328.67..36328.68 rows=1 width=107) (actual time=88957.677..88957.729 rows=15 loops=1) 02 -> Sort (cost=36328.67..36328.68 rows=1 width=107) (actual time=88957.653..88957.672 rows=15 loops=1) 03 Sort Key: dbaj.d_larq, dbaj.c_ajbh 04 Sort Method: top-N heapsort Memory: 27kB 05 -> Nested Loop Semi Join (cost=17099.76..36328.66 rows=1 width=107) (actual time=277.794..88932.662 rows=8605 loops=1) 06 Join Filter: ((dbaj.c_ajbh)::text = (t_zhld_zbajxx.c_ajbh)::text) 07 Rows Removed by Join Filter: 37018710 08 -> Nested Loop (cost=0.00..19200.59 rows=1 width=107) (actual time=199.141..601.845 rows=8605 loops=1) 09 Join Filter: (dbxx.c_bh = dbaj.c_dbbh) 10 Rows Removed by Join Filter: 111865 11 -> Seq Scan on t_zhld_ajdbxx dbaj (cost=0.00..19117.70 rows=219 width=140) (actual time=198.871..266.182 rows=8605 loops=1) 12 Filter: ((n_valid = 1) AND ((c_zblx)::text = '1003'::text) AND ((c_dbfy)::text = '0'::text) AND ((c_gy)::text = '2550'::text)) 13 Rows Removed by Filter: 332691 14 -> Materialize (cost=0.00..66.48 rows=5 width=33) (actual time=0.001..0.017 rows=14 loops=8605) 15 -> Seq Scan on t_zhld_db dbxx (cost=0.00..66.45 rows=5 width=33) (actual time=0.044..0.722 rows=14 loops=1) 16 Filter: ((n_valid = 1) AND ((c_dbztbh)::text = '1003'::text) AND (n_state = ANY ('{1,2,3}'::integer[]))) 17 Rows Removed by Filter: 1297 18 -> Materialize (cost=17099.76..17117.46 rows=708 width=32) (actual time=0.006..4.890 rows=4303 loops=8605) 19 -> HashAggregate (cost=17099.76..17106.84 rows=708 width=32) (actual time=44.011..54.924 rows=8605 loops=1) 20 Group Key: t_zhld_zbajxx.c_ajbh 21 -> Bitmap Heap Scan on t_zhld_zbajxx (cost=163.36..17097.99 rows=708 width=32) (actual time=5.218..30.278 rows=8605 loops=1) 22 Recheck Cond: ((n_dbzt = 1) AND ((c_zblx)::text = '1003'::text)) 23 Filter: ((c_gy)::text = '2550'::text) 24 Rows Removed by Filter: 21849 25 Heap Blocks: exact=960 26 -> Bitmap Index Scan on i_tzhldzbajxx_zblx_dbzt (cost=0.00..163.19 rows=5876 width=0) (actual time=5.011..5.011 rows=30458 loops=1) 27 Index Cond: ((n_dbzt = 1) AND ((c_zblx)::text = '1003'::text)) 28 Planning time: 1.258 ms 29 Execution time: 88958.029 ms 執行計劃解讀： 1：第27->21行，經過索引i_tzhldzbajxx_zblx_dbzt過濾表t_zhld_zbajxx的數據，而後根據過濾條件(c_gy)::text = '2550'::text過濾最終返回8605條數據 2：第17->15行，根據條件過濾t_zhld_db表的數據，最終返回了14條數據 3：第20->19行，對錶t_zhld_zbajxx作group by的操做 4：第13->11行，全表掃描t_zhld_ajdbxx 最終返回了8605條數據