2 where clauses slower than separated queries

Question

I'm using Google Cloud SQL (the micro server version) to run a couple of performance tests.

I want to do the following query:

select count(*) from table where A = valueA and B like "%input_string%";
+----------+
| count(*) |
+----------+
|   512997 |
+----------+
1 row in set (9.64 sec)

If I run them separately, I get:

select count(*) from table where A = valueA;
+----------+
| count(*) |
+----------+
|   512998 |
+----------+
1 row in set (0.18 sec)

select count(*) from table where B like "%input_string%";
+----------+
| count(*) |
+----------+
|   512997 |
+----------+
1 row in set (1.43 sec)

How is that difference in performance possible???

Both A and B columns have indexes as they are used to order tables in a web application.

Thx! EDIT: table schema

table | CREATE TABLE `table` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `A` varchar(9) DEFAULT NULL,
  `B` varchar(50) DEFAULT NULL,
  `C` varchar(10) DEFAULT NULL,
  `D` varchar(50) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `A` (`A`),
  KEY `B` (`B`)
) ENGINE=InnoDB AUTO_INCREMENT=512999 DEFAULT CHARSET=utf8

Answer 1

A option might be using a FULLTEXT INDEX and using MATCH() on it.

CREATE TABLE `table` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `A` varchar(9) DEFAULT NULL,
  `B` varchar(50) DEFAULT NULL,
  `C` varchar(10) DEFAULT NULL,
  `D` varchar(50) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY(A), 
  FULLTEXT INDEX(B)
) ENGINE=InnoDB AUTO_INCREMENT=512999 DEFAULT CHARSET=utf8

And a query rewrite

SELECT 
 count(*)
FROM 
`table`
WHERE
   A = 'A'
 AND 
   B IN (

     SELECT 
      B
     FROM 
      `table`
     WHERE
   MATCH(B) AGAINST('+input_string' IN BOOLEAN MODE)
)

The inner SQL will filter down a possible result based on the FULLTEXT index.
And the outer SQL will do the other filtering.

You could also use a UNION ALL now i think about it.
It should work on this questions CREATE TABLE statement .
The general idea is to get two counts for both filters and pick the lowest as valid count.

Query

SELECT 
 MIN(counted) AS 'COUNT(*)' # Result 512997
FROM (

  select count(*) AS counted from `table` where A = 'A' # Result 512998
  UNION ALL
  select count(*)  from `table` where B like "%input_string%" # Result  512997
) AS counts