sql-server add tag
i-one (imported from SE)
I have a table with a few dozens of rows. Simplified setup is following

    CREATE TABLE #data ([Id] int, [Status] int);
    INSERT INTO #data
    VALUES (100, 1), (101, 2), (102, 3), (103, 2);

And I have a query that joins this table to a set of table value constructed rows (made of variables and constants), like

    DECLARE @id1 int = 101, @id2 int = 105;
    	COALESCE(p.[Code], 'X') AS [Code],
    	COALESCE(d.[Status], 0) AS [Status]
    		(@id1, 'A'),
    		(@id2, 'B')
    	) p([Id], [Code])
    	FULL JOIN #data d ON d.[Id] = p.[Id];

Query execution plan is showing that optimizer's decision is to use `FULL LOOP JOIN` strategy, which seems appropriate, since both inputs have very few rows. One thing I noticed (and cannot agree), though, is that TVC rows are being spooled (see area of the execution plan in the red box).

[![Constant Scan spooling][1]][1]

Why optimizer introduces spool here, what is the reason to do it? There is nothing complex beyond the spool. Looks like it is not necessary. How to get rid of it in this case, what are the possible ways?


The above plan was obtained on

> Microsoft SQL Server 2014 (SP2-CU11) (KB4077063) - 12.0.5579.0 (X64)

  [1]: https://i.stack.imgur.com/sJKdW.png
Top Answer
Paul White (imported from SE)
> Why optimizer introduces spool here, what is the reason to do it? There is nothing complex beyond the spool.

The thing beyond the spool is not a simple table reference, which could simply be duplicated when the left join / anti semi join [alternative][1] is generated.

It may *look* a little like a table (Constant Scan) but to the optimizer[^1] it is a `UNION ALL` of the separate rows in the `VALUES` clause.

The additional complexity is enough for the optimizer to choose to spool and replay the source rows, and not replace the spool with a simple "table get" later on. For example, the initial transformation from full join looks like this:

[![early plan][2]][2]

Notice the extra spools introduced by the general transform. Spools above a simple table get are cleaned up later by the rule `SpoolGetToGet`.

If the optimizer had a corresponding `SpoolConstGetToConstGet` rule, it could work as you wish, in principle.

> How to get rid of it in this case, what are the possible ways?

Use a real table (temporary or variable), or write the transformation from full join manually, for example:

    p([Id], [Code]) AS
        SELECT @id1, 'A'
        UNION ALL
        SELECT @id2, 'B'
    FullJoin AS
        FROM p
        LEFT JOIN #data d 
            ON d.[Id] = p.[Id]
        UNION ALL
        FROM #data AS D
            SELECT *
            FROM p
            WHERE p.Id = D.Id
    COALESCE(FullJoin.Code, 'X') AS Code,
    COALESCE(FullJoin.Status, 0) AS [Status]
FROM FullJoin;

Plan for manual rewrite:

[![Manual rewrite plan][3]][3]

This has an estimated cost of 0.0067201 units, compared with 0.0203412 units for the original.

  [1]: https://blogs.msdn.microsoft.com/craigfr/2006/07/26/nested-loops-join/
  [2]: https://i.stack.imgur.com/iy5Nm.png
  [3]: https://i.stack.imgur.com/lCdoq.png "estimated cost 0.0067201"
  [^1]: It can be observed as a `LogOp_UnionAll` in the *Converted Tree* (TF 8605). In the *Input Tree* (TF 8606) it is a `LogOp_ConstTableGet`. The *Converted Tree* shows the tree of optimizer expression elements after parsing, normalization, algebrization, binding, and some other preparatory work. The *Input Tree* shows the elements after conversion to Negation Normal Form (NNF convert), runtime constant collapsing, and a few other bits and bobs. NNF convert includes logic to collapse logical unions and common table gets, among other things.

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