Re: reloption to prevent VACUUM from truncating empty pages at theend of relation

Hi,

On Mon, Feb 4, 2019 at 1:25 AM Tsunakawa, Takayuki
<tsunakawa.takay@jp.fujitsu.com> wrote:
>
> FYI, it seems that the user sees "shrink" rather than "truncate" in the documentation as below, although these are
aboutVACUUM FULL.
 
>
> https://www.postgresql.org/docs/devel/sql-vacuum.html
> would like the table to physically shrink to occupy less disk space
>
> https://www.postgresql.org/docs/devel/routine-vacuuming.html
> shrink a table back to its minimum size and return the disk space to the operating system,
>
>
>
> Anyway, I don't have any favor about naming this, and I hope native English speakers will choose the best name.  I
won'tobject to whatever name any committer chooses.
 

FWIW, I prefer shrink over truncate, though I'd rather go with
vacuum_shink_enabled as suggested previously.

The patch still applies cleanly and works as intended.  About:

+ * shrink_enabled can be set at ShareUpdateExclusiveLock because it
+ * is only used during VACUUM, which uses a ShareUpdateExclusiveLock,
+ * so the VACUUM will not be affected by in-flight changes. Changing its
+ * value has no affect until the next VACUUM, so no need for stronger lock.

I'm not sure that I get this comment.  Since both require a
ShareUpdateExclusiveLock, you can't change the parameter while a
VACUUM is active on that table.  Did you wanted to use another lock
mode?