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Which is slower to Union or inner selects?
It was performed on Oracle 11g, but I am pretty confident that it applies to most SQL databases. Using the “WHERE” clause after the whole “UNION” is performed is significantly slower than using the “WHERE” clause inside inner selects.
Is it better to use Union all or where clause?
If you can ensure (by inner WHERE clauses) that there will be no duplicates, it’s far better to use UNION ALL and let database engine optimize the inner selects. Using a WHERE clause on the result of grouped results is too expensive because you are operating on more internal results than you need.
Which is better, plain UNION or following Union?
… is better than following which is better than following UNION ALL is faster than UNION because plain UNION is expecting that within two joined datasets are duplicates which need to be removed.
How to improve SQL Server query performance on large tables?
I have a relatively large table (currently 2 million records) and would like to know if it’s possible to improve performance for ad-hoc queries. The word ad-hoc being key here. Adding indexs is not an option (there are already indexs on the columns which are queried most commonly).
Why does SQL Server insert into large table take so long?
Recently I have experimented with changing the clustered index of the large table to an identity int column, which has brought the insert down from 3 hours to one hour, but I am still puzzled why this simple query should take so long to run (regardless of the size of the table) CREATE TABLE [dbo].
Why is my spark executor so slow?
Assuming executors are better provisioned. If you are using Spark’s SQL and the driver is OOM due to broadcasting relations, then either you can increase the driver memory (if possible) or reduce the spark.sql.autoBroadcastJoinThreshold value so that your join operations will use the more memory-friendly sort merge join.