分頁能力跳躍非相鄰頁面無偏移限制

Question

我使用postgres，並試圖分頁結果。 這兩個標準的方法未能滿足我的需求，這是

1. 抵免限額 - 然後我來到這裏是因爲無法跳上非相鄰頁

的 - 因爲成本高抵消

尋求的跨越ROW_NUMBER（）方法。代碼如下

DECLARE @row_per_page INT = 100 
DECLARE @page_number INT = 2 

SELECT * FROM 
(SELECT ROW_NUMBER() OVER (ORDER BY [ID]) AS [RowNumber],* 
FROM table_name) AS T 
WHERE T.[RowNumber] > (@page_number-1)*@row_per_page AND T.[RowNumber] < @page_number*@row_per_page+1 

我不理解這是如何工作的，如果我有一個讓我的結果它仍然會通過整個數據庫搜索，然後將它們分配唯一的行號WHERE條件，然後選擇的一定範圍內row_numbers。 那麼它是如何比偏移限制更好？。有人可以解釋它的工作和表現嗎？

Answer 1

上面引用的代碼實際上比使用LIMIT和OFFSET差，因爲它本質上手工完成了相同的事情，而且代碼很複雜，PostgreSQL不會發現它可以使用部分索引掃描或一個top-N heapsort。

我會告訴你一個例子：

\d large 
    Table "laurenz.large" 
┌────────┬─────────┬───────────┐ 
│ Column │ Type │ Modifiers │ 
├────────┼─────────┼───────────┤ 
│ id  │ integer │ not null │ 
│ val │ text │   │ 
└────────┴─────────┴───────────┘ 
Indexes: 
    "large_pkey" PRIMARY KEY, btree (id) 

這是尋呼與OFFSET和LIMIT做，這取決於你是否有一個索引或不：不帶指數

EXPLAIN (ANALYZE, BUFFERS) 
SELECT * FROM large 
ORDER BY val 
OFFSET 50 LIMIT 10; 
             QUERY PLAN 
------------------------------------------------------------------------------------ 
Limit (cost=52868.58..52868.60 rows=10 width=37) 
     (actual time=1657.981..1658.001 rows=10 loops=1) 
    Buffers: shared hit=8334 
    -> Sort (cost=52868.45..55368.45 rows=1000000 width=37) 
      (actual time=1657.909..1657.950 rows=60 loops=1) 
     Sort Key: val 
     Sort Method: top-N heapsort Memory: 29kB 
     Buffers: shared hit=8334 
     -> Seq Scan on large (cost=0.00..18334.00 rows=1000000 width=37) 
           (actual time=0.010..721.285 rows=1000000 loops=1) 
       Buffers: shared hit=8334 
Planning time: 0.078 ms 
Execution time: 1658.036 ms 

，PostgreSQL必須掃描整個表格，但至少可以發現它只需要前60行。

EXPLAIN (ANALYZE, BUFFERS) 
SELECT * FROM large 
ORDER BY id 
OFFSET 50 LIMIT 10; 
             QUERY PLAN 
----------------------------------------------------------------------------------------- 
Limit (cost=2.14..2.48 rows=10 width=37) 
     (actual time=0.100..0.121 rows=10 loops=1) 
    Buffers: shared hit=4 
    -> Index Scan using large_pkey on large (cost=0.42..34317.43 rows=1000000 width=37) 
              (actual time=0.022..0.073 rows=60 loops=1 
     Buffers: shared hit=4 
Planning time: 0.130 ms 
Execution time: 0.158 ms 

隨着索引，事情是相當快，因爲​​50的偏移足夠小，它不會傷害太多。

現在讓我們嘗試使用row_number同樣的事情：

EXPLAIN (ANALYZE, BUFFERS) 
SELECT id, val 
    FROM (SELECT id, val, 
       row_number() OVER (ORDER BY val) 
     FROM large 
     ) q 
    WHERE row_number BETWEEN 51 AND 60; 
             QUERY PLAN 
---------------------------------------------------------------------------------------- 
Subquery Scan on q (cost=172682.84..205182.84 rows=5000 width=37) 
        (actual time=5663.090..10611.557 rows=10 loops=1) 
    Filter: ((q.row_number >= 51) AND (q.row_number <= 60)) 
    Rows Removed by Filter: 999990 
    Buffers: shared hit=8334, temp read=9770 written=9770 
    -> WindowAgg (cost=172682.84..190182.84 rows=1000000 width=45) 
        (actual time=5662.803..9795.077 rows=1000000 loops=1) 
     Buffers: shared hit=8334, temp read=9770 written=9770 
     -> Sort (cost=172682.84..175182.84 rows=1000000 width=37) 
        (actual time=5662.784..8099.025 rows=1000000 loops=1) 
       Sort Key: large.val 
       Sort Method: external merge Disk: 48848kB 
       Buffers: shared hit=8334, temp read=9770 written=9770 
       -> Seq Scan on large (cost=0.00..18334.00 rows=1000000 width=37) 
             (actual time=0.015..827.945 rows=1000000 loops=1 
        Buffers: shared hit=8334 
Planning time: 0.175 ms 
Execution time: 10621.032 ms 

（14行）

PostgreSQL的排序整個表，然後掃描結果，並扔掉一切，但所需要的行。

EXPLAIN (ANALYZE, BUFFERS) 
SELECT id, val 
    FROM (SELECT id, val, 
       row_number() OVER (ORDER BY id) 
     FROM large 
     ) q 
    WHERE row_number BETWEEN 51 AND 60; 
              QUERY PLAN 
--------------------------------------------------------------------------------------------- 
Subquery Scan on q (cost=0.42..64317.43 rows=5000 width=37) 
        (actual time=0.319..3411.027 rows=10 loops=1) 
    Filter: ((q.row_number >= 51) AND (q.row_number <= 60)) 
    Rows Removed by Filter: 999990 
    Buffers: shared hit=11069 
    -> WindowAgg (cost=0.42..49317.43 rows=1000000 width=45) 
        (actual time=0.040..2585.197 rows=1000000 loops=1) 
     Buffers: shared hit=11069 
     -> Index Scan using large_pkey on large (cost=0.42..34317.43 rows=1000000 width=37) 
                (actual time=0.024..895.798 rows=10 
       Buffers: shared hit=11069 
Planning time: 0.261 ms 
Execution time: 3411.119 ms 

PostgreSQL不必排序，但它仍然掃描整個子查詢結果並拋出大部分行。