Thus, if there are a whole bunch of columns on each table, the data in
those extra columns (e.g. - all columns aside from "id", the one that
was asked for in the result set) will indeed be drawn into memory.
Yeah, I wanted to mean that ;)
All the columns are loaded (except the TOASTed ones which are not
mentioned in the query) into memory, but only the requested ones are
processed and returned to the client...
Is that specific to Postgresql?
Nope. All databases do more or less the same.
From an outside perspective it just
seems odd that potentially a large amount of data would be pulled off
disk into memory that is never used. Perhaps there's an overriding
reason for this.
Yeah, where would you put this data if you didn't put it where it is
now ?
If you alter tables "customer" and "order", taking some columns off,
and stowing them in separate tables, then you'll find that more tuples
of "customer" and "order" will fit into a buffer page, and that the
join will be assembled with somewhat less memory usage.
Whether or not that is a worthwhile change to make will vary
considerably.
Makes designing the schema a bit tough. ;)
"Premature optimization is the root of all evil"
Build a test database, fill it with data, and experiment.
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