DROP PROCEDURE IF EXISTS tree_recover;

DELIMITER //

CREATE PROCEDURE tree_recover ()
MODIFIES SQL DATA
BEGIN

DECLARE currentId, currentParentId  CHAR(36);
DECLARE currentLeft                 INT;
DECLARE startId                     INT DEFAULT 1;

# Determines the max size for MEMORY tables.
SET max_heap_table_size = 1024 * 1024 * 512;

START TRANSACTION;

# Temporary MEMORY table to do all the heavy lifting in,
# otherwise performance is simply abysmal.
CREATE TABLE `tmp_tree` (
    `id`        char(36) NOT NULL DEFAULT '',
    `parent_id` char(36)          DEFAULT NULL,
    `lft`       int(11)  unsigned DEFAULT NULL,
    `rght`      int(11)  unsigned DEFAULT NULL,
    PRIMARY KEY      (`id`),
    INDEX USING HASH (`parent_id`),
    INDEX USING HASH (`lft`),
    INDEX USING HASH (`rght`)
) ENGINE = MEMORY
SELECT `id`,
       `parent_id`,
       `lft`,
       `rght`
FROM   `tree`;

# Leveling the playing field.
UPDATE  `tmp_tree`
SET     `lft`  = NULL,
        `rght` = NULL;

# Establishing starting numbers for all root elements.
WHILE EXISTS (SELECT * FROM `tmp_tree` WHERE `parent_id` IS NULL AND `lft` IS NULL AND `rght` IS NULL LIMIT 1) DO

    UPDATE `tmp_tree`
    SET    `lft`  = startId,
           `rght` = startId + 1
    WHERE  `parent_id` IS NULL
      AND  `lft`       IS NULL
      AND  `rght`      IS NULL
    LIMIT  1;

    SET startId = startId + 2;

END WHILE;

# Switching the indexes for the lft/rght columns to B-Trees to speed up the next section, which uses range queries.
DROP INDEX `lft`  ON `tmp_tree`;
DROP INDEX `rght` ON `tmp_tree`;
CREATE INDEX `lft`  USING BTREE ON `tmp_tree` (`lft`);
CREATE INDEX `rght` USING BTREE ON `tmp_tree` (`rght`);

# Numbering all child elements
WHILE EXISTS (SELECT * FROM `tmp_tree` WHERE `lft` IS NULL LIMIT 1) DO

    # Picking an unprocessed element which has a processed parent.
    SELECT     `tmp_tree`.`id`
      INTO     currentId
    FROM       `tmp_tree`
    INNER JOIN `tmp_tree` AS `parents`
            ON `tmp_tree`.`parent_id` = `parents`.`id`
    WHERE      `tmp_tree`.`lft` IS NULL
      AND      `parents`.`lft`  IS NOT NULL
    LIMIT      1;

    # Finding the element's parent.
    SELECT  `parent_id`
      INTO  currentParentId
    FROM    `tmp_tree`
    WHERE   `id` = currentId;

    # Finding the parent's lft value.
    SELECT  `lft`
      INTO  currentLeft
    FROM    `tmp_tree`
    WHERE   `id` = currentParentId;

    # Shifting all elements to the right of the current element 2 to the right.
    UPDATE `tmp_tree`
    SET    `rght` = `rght` + 2
    WHERE  `rght` > currentLeft;

    UPDATE `tmp_tree`
    SET    `lft` = `lft` + 2
    WHERE  `lft` > currentLeft;

    # Setting lft and rght values for current element.
    UPDATE `tmp_tree`
    SET    `lft`  = currentLeft + 1,
           `rght` = currentLeft + 2
    WHERE  `id`   = currentId;

END WHILE;

# Writing calculated values back to physical table.
UPDATE `tree`, `tmp_tree`
SET    `tree`.`lft`  = `tmp_tree`.`lft`,
       `tree`.`rght` = `tmp_tree`.`rght`
WHERE  `tree`.`id`   = `tmp_tree`.`id`;

COMMIT;

DROP TABLE `tmp_tree`;

END//

DELIMITER ;