Re: delete then insert

On Fri, Jan 17, 2020 at 3:03 PM Ken Benson <Ken(at)infowerks(dot)com> wrote:

>
>
> So – I THINK I know – that when a row is deleted from a table – the row is
> not actually removed from the table – but, merely marked as deleted, thus
> becoming a dead tuple.
> AUTOVACUUM – takes care of the process of removing these dead tuples.
>
>
>
> My question. If – an insert occurs to that same table before autovacuum
> comes along, does that inserted record use a dead tuple? Or – does the
> insert result in an actual added row?
>

There is a double indirection. The index entries point to a specific "line
item" within a specific table block. In that table block, that line item
points to an area in that same block for the actual row data.

Once a row has been delete long enough that no currently active snapshot
can possibly care about it, anyone can "clean up" the table block so that
the "line item" has a flag set to indicate it no longer points to any
storage within the block, and that former storage for the row is then
available for reuse. But only a vacuum can make the line item itself
available for reuse.

Now if someone other than a vacuum has cleaned up a block, will it
advertise the reclaimed space in the freespace map, so that an INSERT can
easily find it? I don't know.

Cheers,

Jeff

>