Why does the same query take different amount of time to run?
That can make a dramatic difference in performance if every read is coming from one of the caches rather than requiring a disk read. If you run the same query later after other work has flushed out most of the blocks your query is interested in, you may end up doing a ton of physical reads rather than being able to use the nicely warmed up cache.
Why do you get different query plans for different executions?
Are you getting different query plans for different executions (you may need to capture a query plan from your initial benchmarks and compare it to the current plan or you may need to increase your AWR retention to retain query plans for a few months in order to see this).
How to force Oracle to use the same query plan?
Depending on the version of Oracle, there are various ways to force Oracle to use the same query plan. If you can drill down and figure out what the problem with statistics is, Oracle probably provides a way to give the optimizer better statistics.
How long does it take power query to load data?
Here’s what the output of this query looks like: On my laptop this query takes around 7-8 seconds to load. Using Process Monitor to see how much data is being read from the file (see this post for more details) shows that Power Query is reading the data from this file twice.
Why are some SQL queries not performing well?
The most basic reason a query doesn’t perform well is because it’s working with too much data. Some queries just have to sift through a lot of data and can’t be helped. That’s unusual, though; most bad queries can be changed to access less data.
What can affect the execution time of a query?
Other factors—such as storage engine locks (table locks and row locks), high concurrency, and hardware—can also have a considerable impact on query execution times.