select * vs select column

Question

If I just need 2/3 columns and I query SELECT * instead of providing those columns in select query, is there any performance degradation regarding more/less I/O

Answer 1

There are reasons for doing things either way. I use SELECT * a lot on PostgreSQL because there are a lot of things you can do with SELECT * in PostgreSQL that you can't do with an explicit column list, particularly when in stored procedures. Similarly in Informix, SELECT * over an inherited table tree can give you jagged rows while an explicit column list cannot because additional columns in child tables are returned as well.

The main reason why I do this in PostgreSQL is that it ensures that I get a well-formed type specific to a table. This allows me to take the results and use them as the table type in PostgreSQL. This also allows for many more options in the query than a rigid column list would.

On the other hand, a rigid column list gives you an application-level check that db schemas haven't changed in certain ways and this can be helpful. (I do such checks on another level.)

As for performance, I tend to use VIEWs and stored procedures returning types (and then a column list inside the stored procedure). This gives me control over what types are returned.

But keep in mind I am using SELECT * usually against an abstraction layer rather than base tables.