SQL SELECT to get the most recently added entries?

Question

I have a database table which is being updated every second. I would like to make write a query that pulls the most recent 20,000 records (about 6 hours worth, since it's collected every second).

I thought to try:

SELECT time_stamp, column2, column3 
FROM myDB.table1 
limit 20000;

But the above query pulls the items from the very beginning of when the data was inserted into the table. I would like the most recent 20000 records.

What would be the query to pull the most recent records inserted into a table?

Answer 1

The correct query is:

SELECT time_stamp, column2, column3
FROM myDB.table1
ORDER BY time_stamp DESC
LIMIT 20000;

However, you might find that the query is expensive. If you have an index on time_stamp , you might find this more efficient:

SELECT time_stamp, column2, column3
FROM myDB.table1
WHERE time_stamp >= date_sub(now(), interval 6 hours);

It is not exactly 20,000 records, but it is the most recent records and it should be faster than sorting all the data.

You can create such an index by doing:

create index idx_table1_timestamp on table1(time_stamp);