提高具有一对多关系的 MYSQL 查询的性能

Question

I have a query in my DB that is taking 25 seconds to return results, which is way too long. It seems like it should be pretty simple. Two tables; the main table (document) is a standard table with some data columns, the join table is a mapping table with only two columns (parent_id, division_id). Previously there wasn't an index on the mapping table so I added one and that changed the "explain" to include the index but doesn't seem to have had an impact on the performance.

The query looks like this:

explain SELECT DISTINCT doc.*  
FROM document doc  
LEFT JOIN multi_division_mapper divisions ON doc.id = divisions.parent_id  
WHERE doc.clientId = 'SOME_GUID'  
AND (divisions.division_id IS NULL OR divisions.division_id  IN ('SOME_GUID'));

and the results of explain are:

Total number of rows in document: 6720 Total number of rows in mapper: 6173

From what I've been able to gather I need to improve either the "type" or the "extra" to make the query faster. What can I do here?

Create table statements:

CREATE TABLE `document` (
   `id` varchar(36) NOT NULL,
   `addedBy` varchar(255) DEFAULT NULL,
   `addedDate` datetime NOT NULL,
   `editedBy` varchar(255) DEFAULT NULL,
   `editedDate` datetime NOT NULL,
   `deleted` bit(1) DEFAULT NULL,
   `clientId` varchar(36) NOT NULL,
   `departmentId` varchar(36) DEFAULT NULL,
   `documentParentId` varchar(36) DEFAULT NULL,
   `documentParent` varchar(50) DEFAULT NULL,
   `fileId` varchar(255) DEFAULT NULL,
   `fileUrl` varchar(600) DEFAULT NULL,
   `documentName` varchar(500) NOT NULL,
   `displayName` varchar(255) NOT NULL,
   `documentId` varchar(45) DEFAULT NULL,
   `notes` varchar(1000) DEFAULT NULL,
   `visibility` varchar(45) NOT NULL DEFAULT 'PRIVATE',
   `documentType` varchar(45) NOT NULL,
   `restrictDelete` bit(1) NOT NULL,
   `customData` text,
   `releaseDate` datetime NOT NULL,
   `expirationDate` datetime NOT NULL,
   `isApproved` bit(1) NOT NULL DEFAULT b'0',
   `userSupplier` varchar(36) DEFAULT NULL,
   `complianceCertificateId` varchar(36) DEFAULT NULL,
   `Status` varchar(50) DEFAULT 'NEUTRAL',
   PRIMARY KEY (`id`),
   KEY `idx_client` (`clientId`)
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8;
 
 CREATE TABLE `multi_division_mapper` (
    `parent_id` varchar(36) NOT NULL,
    `division_id` varchar(36) NOT NULL,
    PRIMARY KEY (`parent_id`,`division_id`),
    KEY `idx_parent` (`parent_id`)
 ) ENGINE=InnoDB DEFAULT CHARSET=latin1;

Answer 1

I was able to get a more favorable EXPLAIN report in a test by creating the following index:

ALTER TABLE multi_division_mapper
  DROP INDEX idx_parent,
  ADD INDEX (division_id, parent_id);

I also dropped idx_parent because it's redundant; it's a prefix of the primary key.

id	select_type	table	partitions	type	possible_keys	key	key_len	ref	rows	filtered	Extra
1	SIMPLE	doc	NULL	ref	idx_client	idx_client	110	const	1	100.00	Using temporary
1	SIMPLE	divisions	NULL	ref	PRIMARY,division_id	division_id	38	const	1	100.00	Using where; Using index; Distinct

The type: ref is better than type: index .

The query I tested is slightly different, but I believe it returns the same result:

SELECT DISTINCT  doc.*   
FROM document doc   
LEFT JOIN multi_division_mapper divisions
  ON doc.id = divisions.parent_id AND divisions.division_id in ('SOME_GUID')  
WHERE doc.clientId = 'SOME_GUID'