Improve performance of MYSQL queries with one-to-many relationships

Question

I have a query in my database that takes 25 seconds to return results, which is too long. It seems like it should be pretty simple. Two tables; the main table (document) is a standard table with some data columns, and the connection table is a mapping table with only two columns (parent_id, division_id). Previously there was no index on the mapping table, so I added an index and changed the "interpretation" to include the index, but it seems to have no impact on performance.

The query is as follows:

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'));

The result of the explanation is:

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

Based on the information I gathered, I need to improve "type" or "extra" to make the query faster. What can I do here?

Create table statement:

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 my tests by creating the following index:

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

I also removed idx_parent because it was redundant; it is a prefix for the primary key.

id	Choose a type	surface	Partition	type	Possible keys	key	key_len	refer to	OK	Filtered	additional
1	Simple	document	null	refer to	idx_client	idx_client	110	constant	1	100.00	Use temporary
1	Simple	department	null	refer to	Main,division_id	Department ID	38	constant	1	100.00	Use location;Use index;Unique