php - 如何优化限制查询以更快地从一个巨大的表中访问数据？

Question

我正在尝试从一个大小为 9 GB 且拥有数百万条记录的表中获取数据。我正在用该数据填充 DataTable。我正在从表中获取记录，即通过 Ajax 和 SQL Limit 查询每页 10 个记录。

pagination

在上图中，您可以看到我们有 223,740 页面，因此当我尝试访问最后一页时，查询需要永远加载数据。但是，当我尝试访问第一页时，数据加载速度更快。但是直接访问更高偏移量的页面需要永远加载。

 public static function getAllEvaluationsWithNameForDataTable($start){
        $queryBuilder = new Builder();

        return  $queryBuilder
            ->from(array('e' =>  static::class))
            ->leftJoin('Cx\Framework\Models\Common\User\CxUser',  'e.cx_hc_user_id = u.id', 'u')
            ->columns('e.id, e.first_name, u.initials as assigned_coach, e.gender, e.email, e.phone, e.age, e.version, e.evaluation_status, e.ip_address, e.date_created, e.date_updated')
            ->orderBy('e.id asc')
            ->limit(10, $start)
            ->getQuery()
            ->execute()
            ->toArray();
}

PHP 函数/ Controller :

public function getEvaluationsAction() {
        // Enable Json response
        $this->setJsonResponse();
        // This action can be called only via ajax
        $this->requireAjax();

        // Forward to access denied if current user is not allowed to view evaluation details
        if (!$this->CxAuth->currentUserIsAllowedTo('VIEW', CxEbEvaluation::getClassResourceName()))
            return $this->forwardToAccessDeniedError();

        if(isset($_GET['start'])){
            $start = $this->request->get('start');
        }else{
            $start = 10;
        }

        $recordsTotal = count(CxEbEvaluation::getAllForDataTable(array('id')));

        //Get Evaluations from DB
        $evaluation_quizzes = CxEbEvaluation::getAllEvaluationsWithNameForDataTable(intval($start));

        //for getting base URL
        $url = new Url();

        $data = array();

        foreach ($evaluation_quizzes as $key => $quiz) {
            $data[ $key ][ 'id' ] = $quiz[ 'id' ];
            $data[ $key ][ 'first_name' ] = $quiz[ 'first_name' ];
            if($quiz[ 'assigned_coach' ]){
                $data[ $key ][ 'assigned_coach' ] = $quiz['assigned_coach'];
            }else{
                $data[ $key ][ 'assigned_coach' ] = "Not assigned";
            }

            $data[ $key ][ 'gender' ] = $quiz[ 'gender' ];
            $data[ $key ][ 'email' ] = $quiz[ 'email' ];
            $data[ $key ][ 'phone' ] = $quiz[ 'phone' ];
            $data[ $key ][ 'age' ] = $quiz[ 'age' ];
            $data[ $key ][ 'version' ] = $quiz[ 'version' ];
            $data[ $key ][ 'quiz' ] =  $url->get('/admin/get-evaluation-quiz-by-id');
            $data[ $key ][ 'manage-notes-messages-and-calls' ] =  $url->get('/admin/manage-notes-messages-and-calls');
            $data[ $key ][ 'date_created' ] = date("m/d/Y H:i:s", $quiz[ 'date_created' ]);
            $data[ $key ][ 'evaluation_status' ] = $quiz[ 'evaluation_status' ];
        }
        // Return data array
        return array(
            "recordsTotal"    => $recordsTotal,
            "recordsFiltered" => $recordsTotal ,
            "data"            => $data //How To Retrieve This Data
        );
        // Return data
    }

Javascript:

cx.common.data.cxAdminDataTables.EbEvaluation = $CxRecordsTable.cxAdminDataTable({
        ajaxUrl: '<?php echo $this->CxHelper->Route('eb-admin-get-evaluations')?>' + eqQuizIdQueryString,
        serverSide: true,
        processing: true,
        recordsFiltered :true,
        columns: [
            cx.common.admin.tableEditColumn('id',{ delete: true }),
            { data: 'first_name' },
            { data: 'assigned_coach' },
            { data: 'gender' },
            { data: 'email' },
            { data: 'phone' },
            { data: 'age' },
            cx.common.admin.tableLinkColumn('quiz', quizLinkOptions),
            cx.common.admin.tableEditColumn('id', healthCoachLinkOptions),
            cx.common.admin.tableLinkColumn('manage-notes-messages-and-calls', manageNotesMessagesAndCalls),
            { data: 'date_created' },
            cx.common.admin.tableSwitchableColumn('evaluation_status', {
                editable: true,
                createdCell: function (td, cellData, rowData, row, col){
                    $(td).data('evaluation-status-id', rowData.id);
                },
                onText: 'Complete',
                offText: 'In progress'
            })
        ],
        toolbarOptions:{
            enabled: false
        },          success: function (data) {
                            cx.common.data.cxAdminDataTables.EbEvaluation.cxAdminDataTable("reloadAjax");
                        }
                    });
                }
                else {
                    $row.removeClass('alert');
                }
            });
        }
    });

我希望问题很清楚。如果需要任何其他信息，请更新我，我会提供。

(来自评论)

SELECT  e.id` AS id, e.first_name AS first_name,
        u.initials AS assigned_coach,
        e.gender AS gender, e.email AS email, e.phone AS phone,
        e.age AS age, e.version AS version,
        e.evaluation_status AS evaluation_status,
        e.ip_address AS ip_address, e.date_created AS date_created,
        e.date_updated AS date_updated
    FROM  evaluation_client AS e
    LEFT JOIN  cx_user AS u  ON e.cx_hc_user_id = u.id
    ORDER BY  e.id ASC
    LIMIT  :APL0 OFFSET, :APL1

Answer 1

Why does MYSQL higher LIMIT offset slow the query down?问题和答案，由 Masivuye Cokile 链接，以及 https://explainextended.com/2009/10/23/mysql-order-by-limit-performance-late-row-lookups/那里提供的链接，包含一个关于为什么大偏移量查询很慢的极好的概述。基本上，对于 LIMIT 150000, 10 MySQL 仍然会扫描整个 150000 行，即使稍后丢弃它们也是如此。要加快速度，您可以:

• 使用顺序分页，即“在 ID #N 后显示 10 个条目”，这种方法运行速度非常快，是一个不错的选择，但会丢弃实际页码；您的用户将看到“下一个/上一个”链接和/或您可以使用 count 查询计算出的大概页码。
• 或者在 id 上创建一个索引，然后强制 mysql 执行仅索引搜索。

对于第二种方法，您必须重写来自

的查询

SELECT ... 
  FROM table t 
WHERE ...
ORDER by t.id ASC
LIMIT 150000, 10

到

SELECT  ...
  FROM (
        SELECT  id
        FROM    table
        ORDER BY
                id ASC
        LIMIT 150000, 10
        ) o
JOIN table t
  ON t.id = o.id
WHERE ...
ORDER BY t.id ASC

或者，由于您不局限于单个查询，您可以使用

检索页面上第一个项目的 ID

SELECT id 
  FROM table 
 ORDER BY id ASC 
 LIMIT 150000, 1

然后使用所述 id 检索实际数据:

SELECT ...
  FROM table
 WHERE id >= $id
   AND ...
 ORDER BY id ASC
 LIMIT 0, 10