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我有一個關於要執行的查詢的問題，但我不知道什麼是最好的性能。我需要將所有單詞排除在與表單詞過濾器關聯的單詞之外。查詢性能 - '左連接爲空'vs'不存在選擇'
查詢的輸出是正確的，但也許有更好的解決方案。我幾乎沒有關於查詢計劃的知識，現在我正試圖理解它。
SELECT CONCAT(SPACE(1), UCASE(stocknews.word.word), SPACE(1)) AS word, stocknews.word.language
FROM stocknews.word
WHERE NOT EXISTS (SELECT word_id FROM stocknews.wordfilter WHERE stocknews.word.id = word_id)
AND user_id = 1
+----+--------------+------------+-------+---------------+---------+---------+-------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | extra |
+----+--------------+------------+-------+---------------+---------+---------+-------+------+-------------+
| 1 | PRIMARY | word | ref | user_id | user_id | 4 | const | 843 | Using where |
| 2 | MATERIALIZED | wordfilter | index | PRIMARY | PRIMARY | 756 | | 16 | Using index |
+----+--------------+------------+-------+---------------+---------+---------+-------+------+-------------+
反對
SELECT CONCAT(SPACE(1), UCASE(stocknews.word.word), SPACE(1)) AS word, stocknews.word.language
FROM stocknews.word
LEFT JOIN stocknews.wordfilter ON stocknews.word.id = stocknews.wordfilter.word_id
WHERE stocknews.wordfilter.word_id IS NULL AND user_id = 1
+----+-------------+------------+------+---------------+---------+---------+---------+------+--------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | extra |
+----+-------------+------------+------+---------------+---------+---------+---------+------+--------------------------------------+
| 1 | SIMPLE | word | ref | user_id | user_id | 4 | const | 843 | |
| 1 | SIMPLE | wordfilter | ref | PRIMARY | PRIMARY | 4 | word.id | 1 | Using where; Using index; Not exists |
+----+-------------+------------+------+---------------+---------+---------+---------+------+--------------------------------------+
歡迎任何幫助！解釋會很好。
編輯：
對於查詢1：
+----------------------------+-------+
| Variable_name | Value |
+----------------------------+-------+
| Handler_commit | 1 |
| Handler_delete | 0 |
| Handler_discover | 0 |
| Handler_external_lock | 0 |
| Handler_icp_attempts | 0 |
| Handler_icp_match | 0 |
| Handler_mrr_init | 0 |
| Handler_mrr_key_refills | 0 |
| Handler_mrr_rowid_refills | 0 |
| Handler_prepare | 0 |
| Handler_read_first | 1 |
| Handler_read_key | 1044 |
| Handler_read_last | 0 |
| Handler_read_next | 859 |
| Handler_read_prev | 0 |
| Handler_read_rnd | 0 |
| Handler_read_rnd_deleted | 0 |
| Handler_read_rnd_next | 0 |
| Handler_rollback | 0 |
| Handler_savepoint | 0 |
| Handler_savepoint_rollback | 0 |
| Handler_tmp_update | 0 |
| Handler_tmp_write | 215 |
| Handler_update | 0 |
| Handler_write | 0 |
+----------------------------+-------+
25 rows in set (0.00 sec)
對於查詢2：
+----------------------------+-------+
| Variable_name | Value |
+----------------------------+-------+
| Handler_commit | 1 |
| Handler_delete | 0 |
| Handler_discover | 0 |
| Handler_external_lock | 0 |
| Handler_icp_attempts | 0 |
| Handler_icp_match | 0 |
| Handler_mrr_init | 0 |
| Handler_mrr_key_refills | 0 |
| Handler_mrr_rowid_refills | 0 |
| Handler_prepare | 0 |
| Handler_read_first | 0 |
| Handler_read_key | 844 |
| Handler_read_last | 0 |
| Handler_read_next | 843 |
| Handler_read_prev | 0 |
| Handler_read_rnd | 0 |
| Handler_read_rnd_deleted | 0 |
| Handler_read_rnd_next | 0 |
| Handler_rollback | 0 |
| Handler_savepoint | 0 |
| Handler_savepoint_rollback | 0 |
| Handler_tmp_update | 0 |
| Handler_tmp_write | 0 |
| Handler_update | 0 |
| Handler_write | 0 |
+----------------------------+-------+
您是否嘗試過並且比較了性能？一般來說（至少在SQL Server中，不確定MariaDB），如果你的表正確索引，'EXISTS'和'NOT EXISTS'執行得更快，因爲它們是短路操作。一旦找到記錄，就會根據您的查詢將其排除或包含在內。 'LEFT JOIN'包括所有的記錄，並且以'IS NULL'標準結尾進行過濾。 –
我在一個小數據集上嘗試了這兩種方法，兩者都有相同的結果。感謝您提供的信息，我幾乎可以確定我的索引是否正確設置。我想我需要製作一個更大的數據集來看看有什麼不同。我的主要問題是聽到查詢使其更快或更慢。所以我猜'NOT EXISTS'方法更快。我將用更大的數據集對其進行測試，並報告實際查詢的速度。謝謝 –
當_word_中有13.000條記錄和_wordfilter_中有5000條記錄時，'NOT EXISTS'方法似乎更快了@ –