Select a random row with where statement is taking to long - mysql

Question

I want to select a random row with a specific where statement but the query is taking to long (around 2.7 seconds)
SELECT * FROM PIN WHERE available = '1' ORDER BY RAND() LIMIT 1
The database contains around 900k rows
Thanks

Answer 1

SELECT * FROM PIN WHERE available = '1' ORDER BY RAND() LIMIT 1
means, that you are going to generate a random number for EVERY row, then sort the whole result-set and finally retrieve one row.
That's a lot of work for querying a single row.
Assuming you have id's without gaps - or only little of them - you better use the programming language you are using to generate ONE random number - and fetch that id:
Pseudo-Example:
result = null;
min_id = queryMinId();
max_id = queryMaxId();
while (result == null){
random_number = random_beetween(min_id, max_id);
result = queryById(randomNumber);
}
If you have a lot of gaps, you could retrieve the whole id-set, and then pick ONE random number from that result prior:
id_set = queryAllIds();
random_number = random_beetween(0, size(id_set)-1);
result = queryById(id_set[random_number])
The first example will work without additional constraints. In your case, you should use option 2. This ensures, that all IDs with available=1 are pre-selected into an 0 to count() -1 array, hence ignoring all invalid ids.
Then you can generate a random number between 0 and count() -1 to get an index within that result-set, which you can translate to an actual ID, which you are going to fetch finally.
id_set = queryAllIdsWithAvailableEqualsOne(); //"Condition"
random_number = random_beetween(0, size(id_set)-1);
result = queryById(id_set[random_number])