[Flask]sqlalchemy使用count()函數遇到的問題

sqlalchemy使用count()函數遇到的問題

在使用flask-sqlalchemy對一個千萬級別表進行count操做時，出現了耗時嚴重、內存飆升的問題。

原代碼：

# 統計當日登錄次數
count = LoginLog.query.filter(LoginLog.username == username, LoginLog.status == 0, db.cast(LoginLog.time, db.DATE) == db.cast(datetime.utcnow(), db.DATE)).count()

sql打印：

SELECT
    count(*) AS count_1
FROM
    (
        SELECT
            loginlog.id AS loginlog_id,
            loginlog.username AS loginlog_username,
            loginlog.time AS loginlog_time,
            loginlog. STATUS AS loginlog_status
        FROM
            loginlog
        WHERE
            loginlog.username = % (username_1) s
        AND loginlog. STATUS = % (status_1) s
        AND CAST(loginlog.time AS DATE) = CAST(%(param_1) s AS DATE)
    ) AS anon_1

進行了一次子查詢，會生成臨時表，效率低。

 

優化代碼：

count = db.session.query(func.count(LoginLog.id)).filter(LoginLog.username == username, LoginLog.status == 0, db.cast(LoginLog.time, db.DATE) == db.cast(datetime.utcnow(), db.DATE)).scalar()

sql打印：

SELECT
    count(loginlog.id) AS count_1
FROM
    loginlog
WHERE
    loginlog.username = % (username_1) s
AND loginlog. STATUS = % (status_1) s
AND CAST(loginlog.time AS DATE) = CAST(%(param_1) s AS DATE)

無子查詢，效率高。