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I have to run a select statement across several tables. I am sure the tables return different records. I am anyway using UNION ALL.
Is it better to use UNION or of UNION ALL in performance terms when I am sure the tables return different records?
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UNION retrieves only distinct records from all queries or tables, whereas UNION ALL returns all the records retrieved by queries. Performance of UNION ALL is higher than UNION.
UNION ALL keeps all of the records from each of the original data sets, UNION removes any duplicate records. UNION first performs a sorting operation and eliminates of the records that are duplicated across all columns before finally returning the combined data set.
The only difference between Union and Union All is that Union extracts the rows that are being specified in the query while Union All extracts all the rows including the duplicates (repeated values) from both the queries.
When comparing UNION vs. UNION ALL , there is one major difference: UNION only returns unique. UNION ALL returns all records, including duplicates.
UNION ALL will perform better than UNION when you're not concerned about eliminating duplicate records because you're avoiding an expensive distinct sort operation. See: SQL SERVER – Difference Between Union vs. Union All – Optimal Performance Comparison
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UNION ALL always is faster, because UNION exclude duplicated entries
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