Satish Burnwal (sburnwal) wrote:
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 ?
yes and no.
a row can consist of both a proper tuple in an 8K block, and toast data
stored in toast tables. the whole block that the tuple is in will be
read into the shared_buffers space, however, toast data thats not
referenced will not be fetched. toast is used for larger fields that
won't fit in a single block.
for more info on toast, see
http://www.postgresql.org/docs/current/static/storage-toast.html
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