query slows down if add field to where condition - mysql

Question

I have a table Mysql fiddle with about 500k records.
CREATE TABLE IF NOT EXISTS `p_transactions` (
`transaction_id` bigint(10) unsigned NOT NULL,
`amount` decimal(19,2) NOT NULL,
`dt` bigint(1) NOT NULL,
`transaction_status` int(1) NOT NULL,
`transaction_type` varchar(15) NOT NULL,
`payment_method` varchar(25) NOT NULL,
`notes` text NOT NULL,
`member_id` int(10) unsigned NOT NULL,
`new_amount` decimal(19,2) NOT NULL,
`paid_amount` decimal(19,2) NOT NULL,
`secret_code` char(40) NOT NULL,
`internal_status` varchar(40) NOT NULL,
`ip_addr` varchar(15) NOT NULL,
`description` text NOT NULL,
`seller_transaction_id` varchar(50) DEFAULT NULL,
`return_url` varchar(255) DEFAULT NULL,
`fail_url` varchar(255) DEFAULT NULL,
`success_url` varchar(255) DEFAULT NULL,
`result_url` varchar(255) DEFAULT NULL,
`user_fee` decimal(19,3) DEFAULT '0.000',
`currency` char(255) DEFAULT 'USD',
`gateway_transaction_id` char(255) DEFAULT NULL,
`load_amount` decimal(19,2) NOT NULL,
`transaction_mode` varchar(1) NOT NULL DEFAULT '',
`p_fee` decimal(19,2) NOT NULL,
`country` varchar(2) NOT NULL,
`email` varchar(255) NOT NULL,
`vat` decimal(19,2) NOT NULL DEFAULT '0.00',
`name` varchar(255) NOT NULL,
`bdate` varchar(255) NOT NULL,
`child_method` varchar(255) NOT NULL,
`processing_fee` decimal(19,2) NOT NULL DEFAULT '0.00',
`flat_fee` varchar(1) NOT NULL DEFAULT 'n',
`user_fee_sum` decimal(19,2) NOT NULL DEFAULT '0.00',
`p_fee_sum` decimal(19,2) NOT NULL DEFAULT '0.00',
`dt_open` bigint(1) NOT NULL DEFAULT '0',
`user_fee_type` varchar(1) NOT NULL DEFAULT 'r',
`custom_gateway_fee` decimal(19,2) NOT NULL DEFAULT '0.00',
`paid_currency` varchar(3) NOT NULL DEFAULT 'USD',
`paid_microtime` bigint(10) unsigned NOT NULL,
`check_ballance` varchar(1) NOT NULL DEFAULT 'n',
PRIMARY KEY (`transaction_id`),
KEY `member_id` (`member_id`),
KEY `payment_method` (`payment_method`),
KEY `child_method` (`child_method`),
KEY `check_ballance` (`check_ballance`),
KEY `dt` (`dt`),
KEY `transaction_type` (`transaction_type`),
KEY `paid_microtime` (`paid_microtime`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
When I execute query
SELECT *
FROM `p_transactions`
WHERE dt >= 1517443200
AND dt <= 1523404799
AND member_id = 2051
ORDER BY `paid_microtime` DESC
LIMIT 50;
it runs 0,000 sec. (+ 0,016 sec. network)
but if I add to query this condition AND transaction_status = 7
SELECT *
FROM `p_transactions`
WHERE dt >= 1517443200
AND dt <= 1523404799
AND member_id = 2051
AND transaction_status = 7
ORDER BY `paid_microtime` DESC
LIMIT 50
query run 12,938 sec. (+ 0,062 sec. network)
Please help me to find out the reason of such behavior
PS. There was index on transaction_status and it increased execution time even more.

Answer 1

Add a suitable index, such as:
ON payzoff_transactions (member_id, dt)
or
ON payzoff_transactions (member_id, dt, transaction_status)
We want member_id column as the leading column in the index, because of the equality comparison, and we expect the result to be a substantially smaller subset of the entire table. We want dt column after that, because of the "range scan" on that.
Including additional columns in the index may allow MySQL to check that condition using values from the index, without a visit/lookup of the row in the underlying table pages.
Either of these indexes would be suitable for both of the queries shown in the question.
Use EXPLAIN to see the execution plan... which index is being used.
There's really no getting around the "Using filesort" operation, since we're pulling a small subset of the entire table.
(If we were pulling the entire table (or a huge subset), we might be able to avoid an expensive sort operation with an access plan that pulls rows in reverse index order, with that has an index with leading column of paid_microtime.)

Answer 2

For the original query have these
INDEX(member_id, dt)
INDEX(member_id, paid_microtime)
For the secondary query, have
INDEX(transaction_status, member_id, dt)
INDEX(transaction_status, member_id, paid_microtime)
Without getting into the details of the distribution of the data values, we cannot explain why one query so much slower; however, my 4 indexes should make both queries run faster most of the time.
More discussion of how I came up with those indexes (and why (member_id, dt, transaction_status) is not so good): http://mysql.rjweb.org/doc.php/index_cookbook_mysql