Sql-server – Referencing another table from a computed column

computed-columnsql server

I am working on a project where many of the entities in the data model do not have a name column, but instead the "name" needs to be constructed from a concatenation of multiple columns. Initially I thought this might be a good use case for a computed column, as it would allow me to define this logic in the database and only once.

However with a fully normalized data model, the computed columns often require values from other tables. Querying other tables can be accomplished through a UDF, but I have read in several places that using UDFs in computed columns prevents parallel execution (see here).

I could use views to handle the formatting of these names, but would like to define the logic only once, and if the logic is in a view it would likely result in nesting views, which can also cause problems with performance.

I'm hoping someone has an idea for how I might define this formatting logic only once in the database without causing a performance problem.

Best Answer

You could use an indexed view for this. Assuming that the relationships are simple (foreign key) relationships, I don't see why this would cause any issues.

Simple example, tested in dbfiddle.uk:

create table game
( game_id int not null primary key,
  game_name varchar(100) not null  
) ;

create table area
( area_id int not null primary key,
  area_name varchar(100) not null 
) ;

create table player
( player_id int not null primary key,
  player_name varchar(100) not null 
) ;

insert into game
values
  (1, 'chess'),
  (2, 'go'),
  (3, 'reversi'),
  (4, 'backgammon'),
  (5, 'hex'),
  (6, 'havannah'),
  (7, 'pacman') ;

insert into area
values
  (11, 'usa'),
  (12, 'russia'),
  (13, 'greece'),
  (14, 'uk'),
  (15, 'france'),
  (16, 'hungary'),
  (17, 'ukraine'),
  (18, 'belgium'),
  (19, 'canada'),
  (20, 'new zealand')  ;

insert into player
values
  (7, 'John'),
  (8, 'Mary'),
  (9, 'Alex'),
  (10, 'Anna'),
  (11, 'Fred'),
  (12, 'Fay') ;

23 rows affected

create table playground
( playground_id int not null identity primary key,
  game_id int not null references game,
  area_id int not null references area,
  player_id int not null  references player,
  various_stuff varchar(100) null default 'abcdefghijklmnopqrstuvwxyz-abcdefghijklmnopqrstuvwxyz-abcdefghijklmnopqrstuvwxyz',
  constraint playground_uq 
    unique (game_id, area_id, player_id)
) ;

create view dbo.play 
  (game_id, area_id, player_id, name)
WITH SCHEMABINDING 
as
select 
    pg.game_id,
    pg.area_id,
    pg.player_id,
    name = g.game_name + '-' + a.area_name + '-' + p.player_name  
from dbo.playground as pg 
  join dbo.game   as g on g.game_id   = pg.game_id 
  join dbo.area   as a on a.area_id   = pg.area_id
  join dbo.player as p on p.player_id = pg.player_id ;

 -- create an index on the view  
create unique clustered index play_cix   
    on play (game_id, area_id, player_id) ;

insert into playground
  (game_id, area_id, player_id)
select game_id, area_id, player_id
from game, area, player ;

420 rows affected

--------------------------------------------------------------------------------
-- Or use XML to see the visual representation, thanks to Justin Pealing and
-- his library: https://github.com/JustinPealing/html-query-plan
--------------------------------------------------------------------------------
set statistics xml on;

select -- top (10) 
   game_id, area_id, player_id, name 
from play 
   WITH (NOEXPAND)            -- Hint used because we are in Express edition
  ;
set statistics xml off;

game_id | area_id | player_id | name                       
------: | ------: | --------: | :--------------------------
      1 |      11 |         7 | chess-usa-John             
      1 |      11 |         8 | chess-usa-Mary             

   --- a few hundred rows omitted

  7 |      20 |        11 | pacman-new zealand-Fred    
  7 |      20 |        12 | pacman-new zealand-Fay     

  
  | Microsoft SQL Server 2005 XML Showplan                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
  | :------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
  | <ShowPlanXML xmlns="http://schemas.microsoft.com/sqlserver/2004/07/showplan" Version="1.8" Build="14.0.3015.40"><BatchSequence><Batch><Statements><StmtSimple StatementText="select -- top (10) &#xa;   game_id, area_id, player_id, name &#xa;from play &#xa;   WITH (NOEXPAND)" StatementId="1" StatementCompId="2" StatementType="SELECT" RetrievedFromCache="true" StatementSubTreeCost="0.00522548" StatementEstRows="420" SecurityPolicyApplied="false" StatementOptmLevel="TRIVIAL" QueryHash="0xBA9BAED6D700FD27" QueryPlanHash="0xE0086846AA2CCE7F" CardinalityEstimationModelVersion="140"><StatementSetOptions QUOTED_IDENTIFIER="true" ARITHABORT="true" CONCAT_NULL_YIELDS_NULL="true" ANSI_NULLS="true" ANSI_PADDING="true" ANSI_WARNINGS="true" NUMERIC_ROUNDABORT="false"></StatementSetOptions><QueryPlan DegreeOfParallelism="0" NonParallelPlanReason="NoParallelPlansInDesktopOrExpressEdition" CachedPlanSize="16" CompileTime="0" CompileCPU="0" CompileMemory="88"><MemoryGrantInfo SerialRequiredMemory="0" SerialDesiredMemory="0"></MemoryGrantInfo><OptimizerHardwareDependentProperties EstimatedAvailableMemoryGrant="419378" EstimatedPagesCached="26211" EstimatedAvailableDegreeOfParallelism="1" MaxCompileMemory="2073864"></OptimizerHardwareDependentProperties><TraceFlags IsCompileTime="1"><TraceFlag Value="8017" Scope="Global"></TraceFlag></TraceFlags><TraceFlags IsCompileTime="0"><TraceFlag Value="8017" Scope="Global"></TraceFlag></TraceFlags><QueryTimeStats ElapsedTime="15" CpuTime="15"></QueryTimeStats><RelOp NodeId="0" PhysicalOp="Clustered Index Scan" LogicalOp="Clustered Index Scan" EstimateRows="420" EstimatedRowsRead="420" EstimateIO="0.00460648" EstimateCPU="0.000619" AvgRowSize="174" EstimatedTotalSubtreeCost="0.00522548" TableCardinality="420" Parallel="0" EstimateRebinds="0" EstimateRewinds="0" EstimatedExecutionMode="Row"><OutputList><ColumnReference Database="[fiddle_9787548beea04054bb68c8990bb6cdab]" Schema="[dbo]" Table="[play]" Column="game_id"></ColumnReference><ColumnReference Database="[fiddle_9787548beea04054bb68c8990bb6cdab]" Schema="[dbo]" Table="[play]" Column="area_id"></ColumnReference><ColumnReference Database="[fiddle_9787548beea04054bb68c8990bb6cdab]" Schema="[dbo]" Table="[play]" Column="player_id"></ColumnReference><ColumnReference Database="[fiddle_9787548beea04054bb68c8990bb6cdab]" Schema="[dbo]" Table="[play]" Column="name"></ColumnReference></OutputList><RunTimeInformation><RunTimeCountersPerThread Thread="0" ActualRows="420" Batches="0" ActualExecutionMode="Row" ActualElapsedms="15" ActualCPUms="15" ActualScans="1" ActualLogicalReads="5" ActualPhysicalReads="0" ActualReadAheads="0" ActualLobLogicalReads="0" ActualLobPhysicalReads="0" ActualLobReadAheads="0" ActualRowsRead="420" ActualEndOfScans="1" ActualExecutions="1"></RunTimeCountersPerThread></RunTimeInformation><IndexScan Ordered="0" ForcedIndex="0" ForceScan="0" NoExpandHint="1" Storage="RowStore"><DefinedValues><DefinedValue><ColumnReference Database="[fiddle_9787548beea04054bb68c8990bb6cdab]" Schema="[dbo]" Table="[play]" Column="game_id"></ColumnReference></DefinedValue><DefinedValue><ColumnReference Database="[fiddle_9787548beea04054bb68c8990bb6cdab]" Schema="[dbo]" Table="[play]" Column="area_id"></ColumnReference></DefinedValue><DefinedValue><ColumnReference Database="[fiddle_9787548beea04054bb68c8990bb6cdab]" Schema="[dbo]" Table="[play]" Column="player_id"></ColumnReference></DefinedValue><DefinedValue><ColumnReference Database="[fiddle_9787548beea04054bb68c8990bb6cdab]" Schema="[dbo]" Table="[play]" Column="name"></ColumnReference></DefinedValue></DefinedValues><Object Database="[fiddle_9787548beea04054bb68c8990bb6cdab]" Schema="[dbo]" Table="[play]" Index="[play_cix]" IndexKind="ViewClustered" Storage="RowStore"></Object></IndexScan></RelOp></QueryPlan></StmtSimple></Statements></Batch></BatchSequence></ShowPlanXML> |

dbfiddle here

and the execution plan of the query that selects from the view, showing that the clustered index on the materialized view is used:

Related Solutions

Sql-server – T-SQL View — How to ‘pre-fetch’ schema using scalar function, then populate using table query

I'm wondering if these views could be re-written using CTEs, to somehow 'pre-fetch' the modified/pre-fixed schemas using the functions, and then query the tables with SELECT INTO to populate data into the view.

No, this isn't possible with a view. You could do something broadly along these lines with a multi-statement function (MSUDF), but:

This would still require a static schema definition for the table variable; and
Careful design would be required to avoid terrible performance; and
All potential base query predicates must be supplied as optional parameters

A multi-statement function would potentially materialize the entire 'view' result set in a table variable, before any predicates in the base query were applied. If the result is large, this overhead will likely be prohibitive.

You could push predicates into the MSUDF using parameters, but then the function body becomes a mess of conditional predicates of the form column = @value OR @value IS NULL, which would require OPTION (RECOMPILE) in the MSUDF to optimize well.

Note that even with the best possible outcome, the users of the view will have to change their queries from view references to MSUDF references (with all parameters specified, DEFAULT or NULL passed for those that are not needed). This may not be workable.

All that said, you need to be certain what is causing the performance problem in the first place. The question does not supply an example of a problematic query plan, so some of what follows is educated guesswork:

There are two features of the current scheme that leap out at me:

The use of ISNULL and calculated columns prevents the optimizer from pushing base-query predicates down into the underlying in-line functions and base tables. You might get better results for the ISNULL part by rewriting the LEFT JOIN as a UNION ALL of an inner join and an anti-semi-join of the two tables. Properly written, this would expose only base column names, allowing the optimizer to push predicates successfully.
The in-line functions use an MSUDF to split the supplied CSV into row values. This is a pattern that often causes optimization problems, because the size and distribution of the result is unknown. A query plan that is optimal for a CSV of '1,2,3' is likely not optimal for a NULL CSV. Again OPTION (RECOMPILE) could help with this. It will simplify the compiled logic for the NULL case, avoid parameter-sniffing issues, as well as providing cardinality information for the MSUDF result. Distribution statistics will still not be available though.

Sql-server – storing as smallint, querying as int

To prove the benefit of data compression for this question, I quickly built the following test:

USE tempdb;

CREATE TABLE dbo.IntTest
(
    IntTest_ID INT NOT NULL
        CONSTRAINT PK_IntTest
        PRIMARY KEY CLUSTERED
)
WITH (DATA_COMPRESSION = PAGE);

CREATE TABLE dbo.SmallIntTest
(
    SmallIntTest_ID SMALLINT NOT NULL
        CONSTRAINT PK_SmallIntTest
        PRIMARY KEY CLUSTERED
)
WITH (DATA_COMPRESSION = NONE);
GO

INSERT INTO dbo.IntTest (IntTest_ID)
SELECT TOP(65535) 
    CONVERT(SMALLINT, ROW_NUMBER() OVER (ORDER BY o.object_id, o1.object_id, o2.object_id) - 32768)
FROM sys.objects o
    , sys.objects o1
    , sys.objects o2
ORDER BY o.object_id, o1.object_id, o2.object_id;

INSERT INTO dbo.SmallIntTest (SmallIntTest_ID)
SELECT TOP(65535) 
    CONVERT(SMALLINT, ROW_NUMBER() OVER (ORDER BY o.object_id, o1.object_id, o2.object_id) - 32768)
FROM sys.objects o
    , sys.objects o1
    , sys.objects o2
ORDER BY o.object_id, o1.object_id, o2.object_id;


SELECT o.name
    , i.name
    , p.partition_number
    , p.rows
    , au.used_pages
    , au.total_pages
    , AvgRowsPerPage = p.rows / CONVERT(DECIMAL(10,2), au.used_pages) 
FROM sys.allocation_units au WITH (NOLOCK)
    INNER JOIN sys.partitions p WITH (NOLOCK) ON ((au.type = 1 OR au.type = 3) AND au.container_id = p.hobt_id) OR (au.type = 2 AND au.container_id = p.partition_id)
    INNER JOIN sys.indexes i ON p.object_id = i.object_id AND p.index_id = i.index_id
    INNER JOIN sys.objects o WITH (NOLOCK) ON p.object_id = o.object_id
    INNER JOIN sys.schemas s WITH (NOLOCK) ON o.schema_id = s.schema_id
WHERE s.name = 'dbo'
    AND o.name IN (
        'IntTest'
        , 'SmallIntTest'
    );

The output from the last query, showing actual pages used by each table, is:

+-----------------+-----------+-------+------------+-------------+-------------+
|      NAME       | PARTITION | ROWS  | USED_PAGES | TOTAL_PAGES | AVG ROWS    |
|                 | NUMBER    |       |            |             | PER PAGE    |
+-----------------+-----------+-------+------------+-------------+-------------+
| PK_IntTest      | 1         | 65535 | 92         | 97          | 712.3369573 |
| PK_SmallIntTest | 1         | 65535 | 92         | 97          | 712.3369573 |
+-----------------+-----------+-------+------------+-------------+-------------+

As you can see from the above results, compressed INTs containing SMALLINTs actually consume precisely the same amount of space.

Best Answer

Related Solutions

Sql-server – T-SQL View — How to ‘pre-fetch’ schema using scalar function, then populate using table query

Sql-server – storing as smallint, querying as int

Related Question