If I make an SQL View with computed columns, but select a subset of those columns, what performance hit am I likely to take

Question

If I have a view in SQL which contains various computed columns, some of which could be very expensive, but only select a few columns out at any one time will I be taking any more of a performance hit, than if I was to separate the views out to multiple views and use them individually? For example if I have 5 columns in my table, and my view contains those same 5 columns, but also 10 simple computed columns and 10 expensive computed columns (using averages or similar) and decide to only select out one or two of the simple computed columns, would I expect that to be any more expensive than if I had separated the expensive columns into their own view? Edit: I'm specifically interested in answers regarding SQL Server and Postgres databases, but a more general response if appropriate will suffice. Edit2: I've looked at query plans in SQL Server and it appears to not bother making a plan for the computed columns when they aren't selected, so I'm hopeful that it's fine to combine all the columns into one view, but would like confirmation :D Edit 3: @NaveenBabu I don't have any performance problems yet - this is somewhat hypothetical. The extra columns will mostly be things like: DATEPART(mm, aDateField), DATEPART(dd, aDateField) ie. simple cheap extensions to the table. But there will be more complicated expensive columns like: (SELECT COUNT(*) FROM events WHERE events.iTicket = tickets.iCode) as NumberOfEvents So I guess if you want a generic example the view would be: CREATE VIEW TicketsView AS SELECT tickets.idx, tickets.a, tickets.b, tickets.c, tickets.d, DATEPART(mm, a) as ticketMonth, DATEPART(dd, a) as ticketDay, DATEPART(yy, a) as ticketYear, (SELECT COUNT(*) FROM events WHERE events.iTicket = tickets.idx) as numEvents FROM tickets Or something like that. The last column is clearly more expensive than the others so: If I SELECT tickets.idx, tickets.b, tickets.ticketMonth FROM TicketsView will it need to do the subselect / count to calculate numEvents, as I haven't selected it out from the view? JavaScript questions and answers, JavaScript questions pdf, JavaScript question bank, JavaScript questions and answers pdf, mcq on JavaScript pdf, JavaScript questions and solutions, JavaScript mcq Test , Interview JavaScript questions, JavaScript Questions for Interview, JavaScript MCQ (Multiple Choice Questions)

Answer 1

In SQL Server the basic principle is that Views are expanded in-line. They're like code-templates that get copied and pasted into your own query. There are other over-heads as well, and you can specify a view not be be expanded in this way, but it's a good general description. One thing that this means is that fields NOT referenced in your query don't get copied though. If a join is needed to derive that column, the join is still necessary - It could duplicate or filter rows from another table, etc - But the scalar calculations would most likely not happen. In your example, using a correlated sub-query for the last field is often slower than a join alternative. But in your case this has a benefit - If you don't select that field, the correlated-sub-query isn't happening. You're introducing a cost when it is selected, and a saving when it's not.