PostgreSQL 查詢效率

Question

我正在使用 PostgreSQL（我是數據庫領域的新手），我想知道您對我在我正在使用的代碼中發現的此類查詢的效率有何看法。 這些查詢有很多 JOIN，其中之一（粗體）根據請求有很多行。 這迫使我們 GROUP BY request.id 以按請求獲取一行和一個包含所有這些行數據的字段（粗體）。

我認為這種查詢必須花費大量時間來尋找所有這些最大值，但我想不出另一種方法來做到這一點。 關於它的效率以及如何改進它的任何想法？

SELECT
  request.id AS id,
  max(request_type.name) AS request_type,
  to_char(max(request.timestamp),'DD/mm/YYYY HH24:mi') AS timestamp,
  to_char(max(request.timestamp),'YYYY-mm-DD') AS timestamp_filtering,
  max(state.name) AS request_state,
  max(users.name || ' ' || COALESCE(users.surname,'')) AS create_user,
  max(request.id_create_user) AS id_create_user,
  max(enterprise.name) AS enterprise,
  max(cause_issue.name) AS cause,
  max(request_movements.id_request_state) AS id_state,
  array_to_string(array_agg(DISTINCT act_code.name || '/' || req_res.act_code), ', ') AS act_code, /* here */
  max(revised.code) AS state_revised, 
  max(request_shipment.warehouse) AS warehouse,
  max(req_res.id_warehouse) AS id_warehouse
FROM
  request
  LEFT JOIN users
    ON users.id=request.id_create_user
  LEFT JOIN enterprise
    ON users.id_enterprise=enterprise.id
  LEFT JOIN request_movements
    ON request_movements.id=request.id_request_movement
  LEFT JOIN request_versions
    ON request_versions.id = request_movements.id_version
  LEFT JOIN state
    ON request_movements.id_request_state=state.id
  INNER JOIN request_type
    ON request.id_request_type=request_type.id
  LEFT JOIN cause_issue
    ON request.id_cause_issue=cause_issue.id
  LEFT JOIN request_reserve req_res
    ON req_res.id_request = request.id /* here */
  LEFT JOIN act_code
    ON req_res.id_act_code=act_code.id
  LEFT JOIN request_shipment
    ON (request_shipment.id_request=request.id)
  LEFT JOIN warehouse_enterprise
    ON (warehouse_enterprise.id = request_shipment.id_warehouse_enterprise)
  LEFT JOIN revised
    ON (revised.id = request_shipment.id_revised)
WHERE
  request.id_request_type = "any_type"  
GROUP BY
  request.id

EXPLAIN 返回this 。

Answer 1

在JOIN大連接之前，您可以通過聚合request_reserve和act_code值來大大簡化此查詢。 這避免了對所有其他列的聚合函數的需要，並且對於大量行通常應該更快。

SELECT r.id
      ,rt.name AS request_type
      ,to_char(r.timestamp, 'DD/mm/YYYY HH24:mi') AS timestamp
      ,to_char(r.timestamp, 'YYYY-mm-DD') AS timestamp_filtering
      ,s.name AS request_state
      ,u.name || COALESCE(' ' || u.surname, '') AS create_user
      ,r.id_create_user
      ,e.name AS enterprise
      ,c.name AS cause
      ,rm.id_request_state AS id_state
      ,rr.act_code
      ,rd.code AS state_revised
      ,rs.warehouse
      ,rr.id_warehouse
FROM      request              r
LEFT JOIN users                u  ON u.id = r.id_create_user
LEFT JOIN enterprise           e  ON e.id = u.id_enterprise
LEFT JOIN request_movements    rm ON rm.id = r.id_request_movement
LEFT JOIN request_versions     rv ON rv.id = rm.id_version
LEFT JOIN state                s  ON s.id = rm.id_request_state
     JOIN request_type         rt ON rt.id = r.id_request_type
LEFT JOIN cause_issue          c  ON c.id = r.id_cause_issue
LEFT JOIN request_shipment     rs ON rs.id_request = r.id
LEFT JOIN warehouse_enterprise w  ON w.id = rs.id_warehouse_enterprise
LEFT JOIN revised              rd ON rd.id = rs.id_revised
LEFT JOIN (
   SELECT rr.id_request, rr.id_warehouse
         ,array_to_string(array_agg(
             DISTINCT a.name || '/' || rr.act_code), ', ') AS act_code
   FROM   request_reserve rr
   LEFT   JOIN act_code   a ON r.id_act_code = a.id
   GROUP  BY rr.id_request, rr.id_warehouse
   )  rr ON rr.id_request = r.id
WHERE  r.id_request_type = "any_type";  -- use single quotes for values!

對於大型查詢，您必須擁有一種人眼可以輕松解析的格式。 因此，我在改進查詢之前重新格式化。 我使用表別名來盡可能避免使用笨拙的標識符。

對create_user小改進：沒有尾隨空格。 如果名稱的任何部分都可以是NULL ，我建議這樣做以避免出現懸空空間：

COALESCE(u.name || ' ' || u.surname, u.name, u.surname)

在 PostgreSQL 9.1 或更高版本中，您可以使用concat_ws() 。