A. Kretschmer wrote:
In response to Satish Burnwal (sburnwal) :
I have a ques - say I have a table that has 10 columns. But in a simple
select query from that table, I use just 3 columns. I want to know
whether even for fetching 3 columns, read happens for all the 10 columns
and out of that the required 3 columns are returned ? ie Does the
complete row with all the 10 columns are fetched even though I need just
3 columns ? OR only 3 columns are fetched ?
Depends, large columns (TEXT, BYTA) are TOASTed¹, that means, the content
of this columns stored in a other table. So it is always a good idea to
specify all rows you needed and don't use select *.
¹ http://www.postgresql.org/docs/8.4/interactive/storage-toast.html
Another reason to prefer explicit column lists over SELECT * for queries
on relations with a lot of columsn and rows, is that it enables some
plan nodes to be more economical with memory, see e.g. comment of
'disuse_physical_tlist':
/*
* disuse_physical_tlist
* Switch a plan node back to emitting only Vars actually referenced.
*
* If the plan node immediately above a scan would prefer to get only
* needed Vars and not a physical tlist, it must call this routine to
* undo the decision made by use_physical_tlist(). Currently, Hash, Sort,
* and Material nodes want this, so they don't have to store useless columns.
*/
regards,
Yeb Havinga
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