ただ，InnoDB はロックの粒度が細かいのはいいんですが，注意しないとデッドロックが
発生してしまうという......そのため，登録処理は単純に START TRANSACTION 〜 COMMIT で
挟むだけじゃダメですね．words テーブルの書き込みは AUTO-INC のロックと行ロックが交錯するし，
行ロックのかかる順番もまちまちなんで，GET_LOCK() 〜 RELEASE_LOCK() で挟んでデッドロック回避．
この GET_LOCK() はテーブルロックとは違うものなので，SELECT での読み出しには影響しません．

CREATE PROCEDURE registurl(urlx varchar(256), mtimex int, totalwordsx int unsigned)
BEGIN
    DECLARE urlid, totaldocs bigint unsigned;
    START TRANSACTION;
    SELECT id INTO urlid FROM urls WHERE url = urlx FOR UPDATE;
    IF urlid IS NOT NULL THEN
        IF GET_LOCK('keywords.words', 10) THEN
            UPDATE regwords, words SET words.df = words.df - 1 WHERE regwords.url_id = urlid AND words.id = regwords.word_id;
            DO RELEASE_LOCK('keywords.words');
            DELETE FROM dispwords WHERE url_id = urlid;
            DELETE FROM regwords WHERE url_id = urlid;
        ELSE
            ROLLBACK;
            TRUNCATE tmpwords;
            SET urlid = NULL, totalwordsx = NULL;
            START TRANSACTION;
        END IF;
    END IF;
    IF totalwordsx IS NULL THEN
        DELETE FROM urls WHERE id = urlid;
        UPDATE count_urls SET n = n - 1 WHERE urlid IS NOT NULL;
        COMMIT;
    ELSE
        DO LAST_INSERT_ID(0);
        INSERT urls (url, mtime, totalwords) VALUES (urlx, FROM_UNIXTIME(mtimex), totalwordsx)
            ON DUPLICATE KEY UPDATE mtime = VALUES(mtime), totalwords = VALUES(totalwords);
        IF urlid IS NULL THEN
            SET urlid = LAST_INSERT_ID();
            UPDATE count_urls SET n = n + 1 WHERE urlid;
        END IF;
        IF urlid && GET_LOCK('keywords.words', 10) THEN
            INSERT words (word) SELECT word FROM tmpwords ON DUPLICATE KEY UPDATE df = words.df + 1;
            DO RELEASE_LOCK('keywords.words');
            UPDATE tmpwords JOIN words USING (word) SET tmpwords.id = words.id, tmpwords.df = words.df;
            INSERT regwords SELECT urlid, id, tf FROM tmpwords;
            SELECT n INTO totaldocs FROM count_urls;
            INSERT dispwords SELECT urlid, id, tf / totalwordsx * (LN(totaldocs / df) + 1) tfidf
                FROM tmpwords WHERE totaldocs / df < 100000 ORDER BY tfidf DESC LIMIT 10;
            COMMIT;
        ELSE
            ROLLBACK;
        END IF;
        TRUNCATE tmpwords;
    END IF;
END