Sql-server – Create index view with self join

This can't be done with a view.

Views in MySQL cannot reference variables. Try defining even a simple view that references a session variable, and there's no joy. Take a working query that references a variable and try creating a view using that query:

ERROR 1351 (HY000): View's SELECT contains a variable or parameter

Granted, there's a hack-around for this: you can call a stored function that you wrote, which returns the value of the variable you want to be accessible to the view. But, oops, we're already out of the scope of "view".

Additionally, a view, like a query, has no ability to iterate through rows and do something in "this" row because of what happened in "that" row.

Granted, again, there are hacks with variables in queries that can emulate some of that behavior but on the best days they can only look backwards and not forwards, and they don't always "see" the rows in the same sequence that the rows are returned.

I see three possible approaches to this, if SQL is where you really want to do it:

Option 1 involves a stored procedure and a temporary table. A stored procedure can iterate through rows with a cursor, and you'd need a temporary table since stored procedures don't have arrays or hashes. Iterate through your source data, either with a cursor or in a loop that starts by finding the top parent node and then each subsequent select based on what you've found so far... populating the temporary table as you go, taking precautions to avoid infinite loops caused by circular references in parent_id values (gotcha!), and then SELECT from your temporary table within the proc to return its contents as a result set to the client.

You might even end up with a second stored procedure that you call from the first one, which then recursively calls itself, to traverse your tree and build your nested set.

Option 2 involves insert, update, and delete triggers on the group and user tables, which would rebuild your MPTT structures every time any modification is done to group and user.

This would, in a sense, be the "most correct" way to do it, since your left and right ids would never be inconsistent with the underlying data... but as @dogmatic69 has pointed out, it's a very expensive operation... and not without its own snags, since you're limited to what you can to to table_x while you're inside a trigger on table_x.

On the other hand, with this option, when you needed to see the tree, the work to build it has already been done... so, much faster on SELECT.

Option 3 is to calculate the tree values with something other than SQL... so, really, I lied earlier, and there were only two SQL options I've come up with. Calculate it all in PHP, Perl, etc., and then populate the database with the values you calculated, which is how I generally do it. :) But I get away with it, because the parent/child relationships in my databases are not updated by any other process. If they were, I'd be staring down the barrel of Option 2.

It seems to ignore any index I put on it

Unless you're using SQL Server Enterprise Edition (or equivalently, Trial and Developer), you will need to use WITH (NOEXPAND) on the view reference in order to use it. In fact, even if you are using Enterprise, there are good reasons to use that hint.

Without the hint, the query optimizer (in Enterprise Edition) may make a cost-based choice between using the materialized view or accessing the base tables. Where the view is as large as the base tables, this calculation may favour the base tables.

Another point of interest is that without a NOEXPAND hint, view references are always expanded to the base query before optimization begins. As optimization progresses, the optimizer may or may not be able to match the expanded definition back to the materialized view, depending on previous optimization activity. This is almost certainly not the case with your simple query, but I mention it for completeness.

So, using the NOEXPAND table hint is your main option, but you might also think about just materializing the base table keys and the columns needed for ordering in the view. Create a unique clustered index on the combined key columns, then a separate nonclustered index on the ordering columns.

This will reduce the size of the materialized view, and limit the number of automatic updates that must be made to keep the view synchronized with the base tables. Your query can then be written to fetch the top 1 keys in the required order from the view (ideally with NOEXPAND), then join back to the base tables to fetch any remaining columns using the keys from the view.

Another variation is to cluster the view on the ordering columns and table keys, then write the query to manually fetch the non-view columns from the base table using the keys. The best option for you depends on the broader context. A good way to decide is to test it with the real data and workload.

Basic solution

CREATE VIEW VI_test
WITH SCHEMABINDING
AS
    SELECT
        t1.PK_ID1,
        t1.something1,
        t1.somethingelse1,
        t2.PK_ID2,
        t2.FK_ID1,
        t2.something2,
        t2.somethingelse2
    FROM dbo.TB_test1 t1
    INNER JOIN dbo.TB_test2 t2 
        ON t1.PK_ID1 = t2.FK_ID1;
GO
-- Brute force unique clustered index
CREATE UNIQUE CLUSTERED INDEX cuq 
ON dbo.VI_test 
    (somethingelse1, somethingelse2, PK_ID1, PK_ID2);
GO
SELECT TOP (1) * 
FROM dbo.VI_test WITH (NOEXPAND)
ORDER BY somethingelse1,somethingelse2;

Execution plan:

Using a nonclustered index

-- Minimal unique clustered index
CREATE UNIQUE CLUSTERED INDEX cuq 
ON dbo.VI_test 
    (PK_ID1, PK_ID2)
WITH (DROP_EXISTING = ON);
GO
-- Nonclustered index for ordering
CREATE NONCLUSTERED INDEX ix 
ON dbo.VI_test (somethingelse1, somethingelse2);

Execution plan:

There is a lookup in this plan, but it is only used to fetch a single row.

Minimal Indexed View

ALTER VIEW VI_test
WITH SCHEMABINDING
AS
    SELECT
        t1.PK_ID1,
        t2.PK_ID2,
        t1.somethingelse1,
        t2.somethingelse2
    FROM dbo.TB_test1 t1
    INNER JOIN dbo.TB_test2 t2 
        ON t1.PK_ID1 = t2.FK_ID1;
GO
-- Unique clustered index
CREATE UNIQUE CLUSTERED INDEX cuq 
ON dbo.VI_test 
    (somethingelse1, somethingelse2, PK_ID1, PK_ID2);

Query:

SELECT TOP (1)
    V.PK_ID1,
    TT1.something1,
    V.somethingelse1,
    V.PK_ID2,
    TT2.FK_ID1,
    TT2.something2,
    V.somethingelse2
FROM dbo.VI_test AS V WITH (NOEXPAND)
JOIN dbo.TB_test1 AS TT1 ON TT1.PK_ID1 = V.PK_ID1
JOIN dbo.TB_test2 AS TT2 ON TT2.PK_ID2 = V.PK_ID2
ORDER BY somethingelse1,somethingelse2;

Execution plan:

This shows the table keys being retrieved (a single row fetch from the view clustered index in order) followed by two single-row lookups on the base tables to fetch the remaining columns.