UNION with WHERE clause

Question

I m doing a UNION of two queries on an Oracle database  Both of them have a WHERE clause  Is there a difference in the performance if I do the WHERE after UNIONing the queries compared to performing the UNION after WHERE clause   For example   SELECT colA  colB FROM tableA WHERE colA  gt  1 UNION SELECT colA  colB FROM tableB WHERE colA  gt  1   compared to   SELECT      FROM  SELECT colA  colB FROM tableA         UNION         SELECT colA  colB FROM tableB    WHERE colA  gt  1   I believe in the second case  it performs a full table scan on both the tables affecting the performance  Is that correct

User · Answer

I would make sure you have an index on ColA  and then run both of them and time them   That would give you the best answer

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i think it will depend on many things - run EXPLAIN PLAN on each one to see what your optimizer selects   Otherwise - as  rayman suggests - run them both and time them

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You need to look at the explain plans  but unless there is an INDEX or PARTITION on COL A  you are looking at a FULL TABLE SCAN on both tables   With that in mind  your first example is throwing out some of the data as it does the FULL TABLE SCAN   That result is being sorted by the UNION  then duplicate data is dropped  This gives you your result set   In the second example  you are pulling the full contents of both tables   That result is likely to be larger   So the UNION is sorting more data  then dropping the duplicate stuff   Then the filter is being applied to give you the result set you are after   As a general rule  the earlier you filter away data  the smaller the data set  and the faster you will get your results    As always  your milage may vary

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SELECT    FROM  SELECT   FROM can     UNION     SELECT   FROM employee  as e WHERE e id   1

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In my experience  Oracle is very good at pushing simple predicates around  The following test was made on Oracle 11 2  I m fairly certain it produces the same execution plan on all releases of 10g as well     Please people  feel free to leave a comment if you run an earlier version and tried the following   create table table1 a number  b number   create table table2 a number  b number    explain plan for select     from  select a b from table1         union          select a b from table2           where a  gt  1   select      from table dbms xplan display format  gt  basic  predicate      PLAN TABLE OUTPUT ---------------------------------------   Id    Operation              Name     ---------------------------------------     0   SELECT STATEMENT                    1    VIEW                               2     SORT UNIQUE                       3      UNION-ALL                        4       TABLE ACCESS FULL  TABLE1       5       TABLE ACCESS FULL  TABLE2   ---------------------------------------  Predicate Information  identified by operation id   ---------------------------------------------------        4 - filter  A  gt 1     5 - filter  A  gt 1    As you can see at steps  4 5   the predicate is pushed down and applied before the sort  union    I couldn t get the optimizer to push down an entire sub query such as   where a    select max a  from empty table    or a join  With proper PK FK constraints in place it might be possible  but clearly there are limitations

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NOTE  While my advice was true many years ago  Oracle s optimizer has improved so that the location of the where definitely no longer matters here   However preferring UNION ALL vs UNION will always be true  and portable SQL should avoid depending on optimizations that may not be in all databases   Short answer  you want the WHERE before the UNION and you want to use UNION ALL if at all possible   If you are using UNION ALL then check the EXPLAIN output  Oracle might be smart enough to optimize the WHERE condition if it is left after   The reason is the following   The definition of a UNION says that if there are duplicates in the two data sets  they have to be removed   Therefore there is an implicit GROUP BY in that operation  which tends to be slow   Worse yet  Oracle s optimizer  at least as of 3 years ago  and I don t think it has changed  doesn t try to push conditions through a GROUP BY  implicit or explicit    Therefore Oracle has to construct larger data sets than necessary  group them  and only then gets to filter   Thus prefiltering wherever possible is officially a Good Idea    This is  incidentally  why it is important to put conditions in the WHERE whenever possible instead of leaving them in a HAVING clause    Furthermore if you happen to know that there won t be duplicates between the two data sets  then use UNION ALL   That is like UNION in that it concatenates datasets  but it doesn t try to deduplicate data   This saves an expensive grouping operation   In my experience it is quite common to be able to take advantage of this operation   Since UNION ALL does not have an implicit GROUP BY in it  it is possible that Oracle s optimizer knows how to push conditions through it   I don t have Oracle sitting around to test  so you will need to test that yourself

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SELECT   FROM  SELECT colA  colB FROM tableA UNION SELECT colA  colB FROM tableB  as tableC WHERE tableC colA  gt  1   If we re using a union that contains the same field name in 2 tables  then we need to give a name to the sub query as tableC in above query   Finally  the WHERE condition should be WHERE tableC colA  gt  1

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Just a caution  If you tried  SELECT colA  colB FROM tableA WHERE colA  gt  1 UNION SELECT colX  colA FROM tableB WHERE colA  gt  1   compared to   SELECT      FROM  SELECT colA  colB FROM tableA         UNION         SELECT colX  colA FROM tableB    WHERE colA  gt  1   Then in the second query  the colA in the where clause will actually have the colX from tableB  making it a very different query  If columns are being aliased in this way  it can get confusing

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SELECT colA  colB FROM tableA  WHERE colA  gt  1 UNION SELECT colX  colA FROM tableB

[sql] UNION with WHERE clause

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