Question

I have a query with a long chain of CTEs which ends with

SELECT RegionName, AreaName, CityName, SubCityName, StreetName 
FROM tDictionaryStreets
UNION ALL
SELECT RegionName, AreaName, CityName, SubCityName, StreetName 
FROM tDictionaryRegions

The execution time of this query is 1450 ms. When I execute these 2 SELECTs separatly it takes much less time. For the query

SELECT RegionName, AreaName, CityName, SubCityName, StreetName 
FROM tDictionaryStreets

execution time is 106 ms. And for the query

SELECT RegionName, AreaName, CityName, SubCityName, StreetName 
FROM tDictionaryRegions

it’s 20 ms.

Why UNION ALL increases the execution time in more than 10 times? What can I do to decrease it?

Thank you for your help.

UPDATED
The whole query (I shortened it, but the problem still presents) is

WITH tFoundRegions AS
(
    SELECT KladrItemName FROM dbo.tBuiltKladrItemsWithQuants
    WHERE UserID = @UserID AND (indeces & 1) > 0
),
tFoundAreas AS
(
    SELECT KladrItemName FROM dbo.tBuiltKladrItemsWithQuants
    WHERE UserID = @UserID AND (indeces & 2) > 0
),
tFoundCities AS
(
    SELECT KladrItemName FROM dbo.tBuiltKladrItemsWithQuants
    WHERE UserID = @UserID AND (indeces & 4) > 0
),
tFoundSubCities AS
(
    SELECT KladrItemName FROM dbo.tBuiltKladrItemsWithQuants
    WHERE UserID = @UserID AND (indeces & 8) > 0
),
tFoundStreets AS
(
    SELECT KladrItemName FROM dbo.tBuiltKladrItemsWithQuants
    WHERE UserID = @UserID AND (indeces & 16) > 0
),
tDictionaryStreets AS
(
    SELECT DISTINCT
        CASE WHEN RegionName IN (SELECT KladrItemName FROM tFoundRegions) THEN RegionName ELSE NULL END RegionName
      , CASE WHEN AreaName IN (SELECT KladrItemName FROM tFoundAreas) THEN AreaName ELSE NULL END AreaName
      , CASE WHEN CityName IN (SELECT KladrItemName FROM tFoundCities) THEN CityName ELSE NULL END CityName
      , CASE WHEN SubCityName  IN (SELECT KladrItemName FROM tFoundSubCities) THEN SubCityName ELSE NULL END SubCityName
      , StreetName 
    FROM StreetNames
    WHERE StreetName IN (SELECT KladrItemName FROM tFoundStreets)
),
tMissingSubCities AS
(
    SELECT KladrItemName FROM tFoundSubCities
    WHERE KladrItemName NOT IN (SELECT SubCityName FROM tDictionaryStreets)
),
tDictionarySubCities AS
(
    SELECT DISTINCT 
        CASE WHEN RegionName IN (SELECT KladrItemName FROM tFoundRegions) THEN RegionName ELSE NULL END RegionName
      , CASE WHEN AreaName IN (SELECT KladrItemName FROM tFoundAreas) THEN AreaName ELSE NULL END AreaName
      , CASE WHEN CityName IN (SELECT KladrItemName FROM tFoundCities) THEN CityName ELSE NULL END CityName
      , SubCityName
      , NULL StreetName 
    FROM SubCityNames
    WHERE SubCityName IN (SELECT KladrItemName FROM tMissingSubCities)
)
SELECT RegionName, AreaName, CityName, SubCityName, StreetName 
FROM tDictionaryStreets
UNION ALL
SELECT RegionName, AreaName, CityName, SubCityName, StreetName 
FROM tDictionarySubCities

Answer 1

Make sure you clear the execution + data caches between each test run.

e.g.

DBCC FREEPROCCACHE
DBCC DROPCLEANBUFFERS

If you run with the UNION ALL first, and then run the 2 selects separately afterwards, the data will already be cached in memory making performance much better (therefore giving the false impression that the subsequent approach is quicker when it may not be).

If you used a UNION then that may well be slower as it has to apply a DISTINCT, but UNION ALL doesn’t have to do that so it should be no different.

Update:
Have a look at the execution plans and compare them – see if there is any difference. You can view the execution plan by clicking the “Include Actual Execution Plan” button in SSMS before running the query

Update 2:

Based on full CTEs given, I think I’d be looking at optimising those – I don’t think the UNION ALL is actually the problem.

IMHO, best thing to try is work through the CTEs one by one and try to optimise each one individually so that when you then combine them all in the main query, they perform better.

e.g. for tDictionaryStreets, how about trying this:

SELECT DISTINCT
    r.KladrItemName AS RegionName,
        a.KladrItemName AS AreaName,
        c.KladrItemName AS CityName,
        sc.KladrItemName AS SubCityName,
        s.StreetName      
FROM StreetNames s
    JOIN tFoundStreets fs ON s.StreetName = fs.KladrItemName
    LEFT JOIN tFoundRegions r ON s.RegionName = r.KladrItemName
    LEFT JOIN tFoundAreas a ON s.AreaName = a.KladrItemName
    LEFT JOIN tFoundCities c ON s.CityName = c.KladrItemName
    LEFT JOIN tFoundSubCities sc ON s.SubCityName = scc.KladrItemName

KladrItemName on each table should at least have an index on.
Try reworking tDictionarySubCities in the same kind of way with joins too.