Question

I’m looking at an execution plan from a troublesome query.

I can see that 45% of the plan is taken up doing a table scan on a table with seven (7) rows of data.

I am about to put a clustered index to cover the columns in my query on a table with seven rows and it feels…wrong. How can this part of my query take up so much of the plan given the table is so tiny?

I was reading up here and it feel it might just be becuase of non-contiguous data – there are no indexes at all on the table in question. Overall though our database is large-ish (7GB) and busy.

I’d love to know what others think – thanks!

EDIT:

The query is run very frequently and was involved in deadlock (and chosen as the victim). Right now it’s taking between 300ms and 500ms to run, but will take longer when the database is busier.

The query:

select l.team1Score, l.team2Score, ls.team1ExternalID, ls.team2ExternalID, et.eventCategoryID, e.eventID, ls.statusCode 
from livescoretracking l(nolock) 
inner join liveScores ls (nolock) on l.liveScoreID = ls.liveScoreID 
inner join db1.dbo.events e on e.gameid = ls.gameid 
inner join db1.dbo.eventtype et (nolock) on e.eventTypeID = et.eventTypeID 
inner join eventCategoryPayTypeMappings ecb (nolock) on ( et.eventCategoryID = ecb.eventCategoryID and e.payTypeID = ecb.payTypeID and ecb.mainEvent = 1 ) 
where ls.gameID = 286711 order by l.dateinserted

The problem table is the eventCategoryPayTypeMappings table – thanks!

Answer 1

A percentage cost is meaningless without knowing the total cost in real terms. e.g. if the query takes 1 ms to execute a 45% cost for a table scan is .45 of a milisecond which is not worth trying to optimise, if the query takes 10 seconds to execute then the 45% cost is significant and worth optimising.