When does a previously-fast SQL query starts running slow?
Fortunately, it doesn’t happen that often. Look at the query plan for the query (when it is running slow). I am guessing you will find a nested loop join occurring one or more times on tables with no indexes for the join. This really slows things down.
Why is Management Studio Query slow in SQL Server?
Client applications setting ARITHABORT to OFF can receive different query plans making it difficult to troubleshoot poorly performing queries. That is, the same query can execute fast in management studio but slow in the application. Another interesting topic to understand is Parameter Sniffing as outlined in Slow in the Application, Fast in SSMS?
Why is running a query on SQL Azure so much slower?
The performance of individual queries is heavily affected by the performance tiers. I know the documentation implies the tiers are about load, that is not strictly true. I would re-run your test with an S2 database as a starting point and go from there.
Is it slow to use subqueries in MySQL?
Subqueries execute every time you evaluate them (in MySQL anyway, not all RDBMSes), i.e. you’re basically running 7 million queries! Using a JOIN, if possible, will reduce this to 1. Even if adding indexing improves performance of those, you’re still running them. Yes, IN with subqueries is slow. Use a join instead.
Why is SQL Server so slow on SSMS?
Slow in the Application, Fast in SSMS? Understanding Performance Mysteriesis another excellent reference. I’ve concluded (perhaps incorrectly) from these little experiments that the reason for the slow-down is due to how SQL’s cached execution plan is set up — when the query is a little different, it has to create a new execution plan.
How long does MySQL ” in ” query take to complete?
I have a MySQL query (Ubu 10.04,Innodb, Core i7, 16Gb RAM, SSD drives, MySQL params optimized): The table em_link_data has about 7million rows, em_link has a few thousand. This query will take about 18 seconds to complete. However, if I substitute the results of the subquery and do this: then the query will run in less than 1 millisecond.
Why are composite indexes so slow in MySQL?
This also means that by the time it does the created_at match, it will only have the rows with status ‘error’, so the dataset it needs to go through is considerably smaller than in the original query. Duration query is not doing so good, we can see MySQL picked the created_at index which we added, but the query itself still takes around 1.6s.
How to increase query speed with index hints?
As you can see, using index hints can further increase the query speed. The difference in USE INDEX And FORCE INDEX comes from (Quote from MySQL documentation): The FORCE INDEX hint acts like USE INDEX ( index_list), with the addition that a table scan is assumed to be very expensive.
Why is the duration query in MySQL slow?
Duration query is not doing so good, we can see MySQL picked the created_at index which we added, but the query itself still takes around 1.6s. So we are stuck again, we added indexes for all the columns used in the WHERE clause but the query is still slow, now what?