How to query a table efficiently without affecting live transactions I have a high transaction table with millions of records with the structure below:

transaction
--------------
id             int
txn_status     varchar
amount         bigint
name           varchar
txn_time       datetime --Date and time of the transaction


The txn_status field value can be completed or Pending. I need to run a query that will retrieve the number of records with the following characteristics:


Transactions with Pending txn_status since the last 15 minutes i.e (current_time - txn_time) &lt;= 15 minutes
Transactions with Pending txn_status between the last 16 and 25 minutes i.e (current_time - txn_time) &gt;= 16 minutes and (current_time - txn_time) &lt;= 25 minutes
Transactions with Pending txn_status between the last 26 and 30 minutes i.e (current_time - txn_time) &gt;= 26 minutes and (current_time - txn_time) &lt;= 30 minutes
Transactions with Pending txn_status more than 30 minutes i.e (current_time - txn_time) &gt; 30 minutes


There are two solutions currently in my head currently:


Query the production database every minute using a case statement (the case statement will filter and group the records based on the time difference above) in the where clause of the query.
To avoid putting much load on the production database, have a separate database with a similar table and have an update and insert triggers update the table and execute the query in No. 1 above afterwards on the database.


Please if there is any optimal or better solution in achieving this without putting much load on the database, please share.
Typically, you don't want to be querying tables with high transaction volume at all; that's what a data warehouse is for.

But if you must, you can use SNAPSHOT ISOLATION. Which will query the table as of the last committed transaction before it started. This might give you dirty reads, as it may be out of date by the time the query finished running, but as you're looking at stuff 15-30 minutes in the past it should be ok.

In MySQL (assuming you're using innoDB), you can use START TRANSACTION WITH CONSISTENT SNAPSHOT; as there is no global setting for SNAPSHOT ISLOATION.

You can also look here for more info: https://dev.mysql.com/doc/refman/5.7/en/innodb-consistent-read.html