我正在使用 PostgreSQL(我是数据库领域的新手),我想知道您对我在使用的代码中发现的此类查询的效率的看法。这些查询有很多 JOIN,其中一个(粗体)按请求有很多行。这迫使我们 GROUP BY request.id 以便按请求获取行和包含所有这些行数据的字段(粗体)。
我认为这种查询必须花费大量时间来寻找所有这些最大值,但我想不出另一种方法来做到这一点。关于其效率以及如何改进它的任何想法?
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
解释返回this。