| From: | Gokulakannan Somasundaram <gokul007(at)gmail(dot)com> |
|---|---|
| To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
| Cc: | Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, Simon Riggs <simon(at)2ndquadrant(dot)com>, Kevin Grittner <Kevin(dot)Grittner(at)wicourts(dot)gov>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: the big picture for index-only scans |
| Date: | 2011-08-19 18:06:35 |
| Message-ID: | CAHMh4-ZQGRQvFVDnUy7LWcTEsmLk61F+qtCOCLEvQNVXQCyz9g@mail.gmail.com |
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| Lists: | pgsql-hackers |
>
>> Well, that would certainly be alarming if true, but I don't think it
>> is. As far as I can see, the overhead of making the visibility map
>> crash-safe is just (1) a very small percentage increase in the work
>> being done by VACUUM and (2) a slight possibility of extra work done
>> by a foreground process if the visibility map bit changes at almost
>> exactly the same time the process was about to insert, update, or
>> delete a tuple.
>>
>> Let's forget the overhead posed by vacuum. Can you please point me to the
> design which talks in detail of the second overhead?
>
> Thanks.
>
If you are following the same design that Heikki put forward, then there is
a problem with it in maintaining the bits in page and the bits in visibility
map in sync, which we have already discussed.
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