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Editorial Team
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Asked: 2026-05-18T05:28:44+00:00

I’m having a hard time getting better results on this query: I have the

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I’m having a hard time getting better results on this query:

I have the following 2 tables:

DROP TABLE IF EXISTS `casino`.`mutualfriends`;
CREATE TABLE  `casino`.`mutualfriends` (
  `CustUID` varchar(64) CHARACTER SET ascii NOT NULL,
  `CustUID2` varchar(64) CHARACTER SET ascii NOT NULL,
  `FType` tinyint(4) NOT NULL,
  PRIMARY KEY (`CustUID`,`CustUID2`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8;


DROP TABLE IF EXISTS `casino`.`customers`;
CREATE TABLE  `casino`.`customers` (
  `CustFullName` varchar(45) NOT NULL,
  `CustEmail` varchar(128) NOT NULL,
  `CustUID` varchar(64) CHARACTER SET ascii NOT NULL,
  `CustMoney` bigint(20) NOT NULL DEFAULT '0',
  `SmallPicURL` varchar(120) CHARACTER SET ascii DEFAULT '',
  `LargePicURL` varchar(120) CHARACTER SET ascii DEFAULT '',
  PRIMARY KEY (`CustUID`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

The customers table has 1M rows, mutual friends has 500K rows.

I’m having a hard time optimizing this query; it seems like there is a table scanning using it. I would like to minimize the scan:

SELECT c.CustUID AS Cuid, c.CustFullName, c.CustMoney, c.SmallPicURL
FROM `customers` c
WHERE c.`CustUID` = '1:1542073175'
UNION
SELECT m.`CustUID2` AS Cuid, c.CustFullName, c.CustMoney, c.SmallPicURL
FROM `mutualfriends` m, `customers` c
WHERE m.`CustUID` = '1:1542073175'
AND c.`CustUID` = m.`CustUID2`
UNION
SELECT m.`CustUID` AS Cuid, c.CustFullName, c.CustMoney, c.SmallPicURL
FROM `mutualfriends` m, customers c
WHERE m.`CustUID2` = '1:1542073175'
AND c.CustUID = m.`CustUID`
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1 Answer

  1. Editorial Team

    I think you need an index on MutualFriends.CustUID2 (a duplicates index, not a unique index; and not as part of the primary key). The primary key probably gives you a usable index for the queries where the identified customer (‘1:1542073175’) is listed first.

    Check the explanation of the query plan.

    I think you’re right that there is a sequential scan for the third part of the UNION, and possibly the second too.

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