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I think this is a pretty common scenario: I have a webpage that’s returning links and excerpts to the 10 most recent blog entries.
If I just queried the entire table, I could use my ORM mapped object, but I’d be downloading all the blog text.
If I restricted the query to just the columns that I need, I’d be defining another class that’ll hold just those required fields.
How bad is the performance hit if I were to query entire rows? Is it worth selecting just what I need?
The answer is “it depends”.
There are two things that affect performance as far as column selection:
Are there covering indexes? E.g. if there is an index containing ALL of the columns in the smaller query, then a smaller column set would be extremely benefifical performance wise, since the index would be read without reading any rows themselves.
Size of columns. Basically, count how big the size of the entire row is, vs. size of only the columns in smaller query.
If the ratio is significant (e.g. full row is 3x bigger), then you might have significant savings in both IO (for retrieval) and network (for transmission) cost.
If the ratio is more like 10% benefit, it might not be worth it as far as DB performance gain.