44 messages in org.postgresql.pgsql-hackersRe: New feature request: FlashBack Query| From | Sent On | Attachments |
|---|---|---|
| RPK | Feb 17, 2007 6:49 am | |
| Joshua D. Drake | Feb 17, 2007 7:50 am | |
| Tom Lane | Feb 17, 2007 8:48 am | |
| elein | Feb 17, 2007 4:06 pm | |
| Chad Wagner | Feb 17, 2007 4:43 pm | |
| Joshua D. Drake | Feb 17, 2007 7:21 pm | |
| Chad Wagner | Feb 17, 2007 7:49 pm | |
| Tom Lane | Feb 17, 2007 9:31 pm | |
| Warren Turkal | Feb 17, 2007 10:46 pm | |
| Hannu Krosing | Feb 18, 2007 1:45 pm | |
| Joshua D. Drake | Feb 18, 2007 2:27 pm | |
| Andreas 'ads' Scherbaum | Feb 19, 2007 1:53 am | |
| Hannu Krosing | Feb 19, 2007 4:36 am | |
| Florian G. Pflug | Feb 19, 2007 6:13 am | |
| Alvaro Herrera | Feb 19, 2007 6:27 am | |
| Zeugswetter Andreas ADI SD | Feb 19, 2007 6:32 am | |
| Zeugswetter Andreas ADI SD | Feb 19, 2007 6:38 am | |
| Florian G. Pflug | Feb 19, 2007 6:59 am | |
| Gregory Stark | Feb 19, 2007 7:18 am | |
| tom...@tuxteam.de | Feb 19, 2007 8:18 am | |
| August Zajonc | Feb 19, 2007 10:09 am | |
| Florian G. Pflug | Feb 19, 2007 11:30 am | |
| August Zajonc | Feb 19, 2007 12:00 pm | |
| Tom Lane | Feb 19, 2007 7:39 pm | |
| Jonah H. Harris | Feb 19, 2007 8:53 pm | |
| Gregory Stark | Feb 20, 2007 12:58 am | |
| RPK | Feb 20, 2007 3:25 am | |
| Andrew Dunstan | Feb 20, 2007 4:42 am | |
| Jonah H. Harris | Feb 20, 2007 7:19 am | |
| Rod Taylor | Feb 20, 2007 7:42 am | |
| Hannu Krosing | Feb 20, 2007 8:02 am | |
| Gregory Stark | Feb 20, 2007 8:28 am | |
| August Zajonc | Feb 20, 2007 8:39 am | |
| RPK | Feb 20, 2007 10:27 am | |
| Tom Lane | Feb 20, 2007 10:40 am | |
| Theo Schlossnagle | Feb 20, 2007 10:45 am | |
| Jonah H. Harris | Feb 20, 2007 11:48 am | |
| Jonah H. Harris | Feb 20, 2007 12:03 pm | |
| August Zajonc | Feb 20, 2007 7:30 pm | |
| Csaba Nagy | Feb 21, 2007 1:17 am | |
| Florian G. Pflug | Feb 21, 2007 6:01 am | |
| Alvaro Herrera | Feb 21, 2007 6:13 am | |
| Florian G. Pflug | Feb 21, 2007 7:08 am | |
| August Zajonc | Mar 1, 2007 9:48 am |
| Subject: | Re: New feature request: FlashBack Query | |
|---|---|---|
| From: | Hannu Krosing (han...@skype.net) | |
| Date: | Feb 19, 2007 4:36:18 am | |
| List: | org.postgresql.pgsql-hackers | |
Ühel kenal päeval, P, 2007-02-18 kell 14:27, kirjutas Joshua D. Drake:
Hannu Krosing wrote:
Ühel kenal päeval, L, 2007-02-17 kell 22:49, kirjutas Chad Wagner:
However, they don't have vacuum, we do.
Right, and I think that is more or less because Oracle doesn't need it. Vacuum's main purpose (correct me if I am wrong) is to recover/mark rows that are no longer used, and Oracle essentially reuses the space immediately.
Obviously with Oracle if you bloat out a table and delete a ton of rows then you have to rebuild the table, but that is more or less the same problem that PostgreSQL has and where vacuum full comes into play.
The only benefit with the Oracle model is that you can achieve flashback, which is a very rarely used feature in my book.
We can have flashbacks up to the last vacuum. It is just not exposed. Don't vacuum, and you have the whole history. (Actually you can't go for more than 2G transactions, or you get trx id rollover).
To get a flashback query, you "just" have to construct a snapshot from that time and you are done. We don't store transaction times anywere, so the flashback has to be by transaction id, but there is very little extra work involved. We just don't have syntax for saying "SELECT ... AS SEEN BY TRANSACTION XXX"
Well this is certainly interesting. What do we think it would take to enable the functionality?
First we must run the query in serializable mode and replace the snapshot with a synthetic one, which defines visibility at the start of the desired transaction
probably it is a good idea to take a lock on all tables involved to avoid a vacuum to be started on them when the query is running.
also, we can't trust the DELETED flags in index pages, so we should forbid index scans, or just always re-check the visibility in heap.
Otherways it would probably be enough to just scan tuples as usual, and check if they were visible to desired transaction, that is they were inserted before that transaction and they are not deleted before that trx.
Of course this will not be true, once we have HOT/WIP with in-page vacuuming.
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