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

Just to add a nuance to the discussion which I don't see here: In terms of I/O, if you're using a database with column-oriented storage you can do A LOT less I/O if you only query for certain columns. As we move to SSDs the benefits may be a bit smaller vs. row-oriented storage but there's a) only reading the blocks that contain columns you care about b) compression, which generally greatly reduces the size of the data on disk and therefore the volume of data read from disk.

If you're not familiar with column-oriented storage, one implementation for Postgres comes from Citus Data, another is Greenplum, another Paraccel, another (loosely speaking) is Amazon Redshift. For MySQL there's Infobright, the now-nigh-defunct InfiniDB. Other commercial offerings include Vertica from HP, Sybase IQ, Teradata...