Add cursors outside transactions thread.
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Bruce Momjian
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doc/TODO.detail/cursor
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530
doc/TODO.detail/cursor
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To: Hiroshi Inoue <Inoue@tpf.co.jp>
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cc: Bruce Momjian <pgman@candle.pha.pa.us>,
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Florian Wunderlich <fwunderlich@devbrain.de>, pgsql-general@postgresql.org
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Subject: Re: [GENERAL] persistent portals/cursors (between transactions)
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In-Reply-To: <3C510D24.8E1FDF7F@tpf.co.jp>
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References: <200201250319.g0P3Jq022575@candle.pha.pa.us> <23244.1011932544@sss.pgh.pa.us> <3C510D24.8E1FDF7F@tpf.co.jp>
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Comments: In-reply-to Hiroshi Inoue <Inoue@tpf.co.jp>
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message dated "Fri, 25 Jan 2002 16:45:40 +0900"
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Date: Fri, 25 Jan 2002 10:12:51 -0500
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Message-ID: <25361.1011971571@sss.pgh.pa.us>
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From: Tom Lane <tgl@sss.pgh.pa.us>
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Hiroshi Inoue <Inoue@tpf.co.jp> writes:
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> Tom Lane wrote:
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>> If it's not holding any locks, I can guarantee you it's not insensitive.
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>> Consider VACUUM, or even DROP TABLE.
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> It's already possible to keep a lock accross transactions.
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> So it would keep an AccessShareLock across transactions.
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AccessShareLock would fend off DROP/ALTER TABLE, but not VACUUM anymore.
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We'd need to invent Yet Another lock type that would prevent VACUUM.
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Clearly that's perfectly doable.
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But: having just finished a lot of work to ensure that VACUUM could run
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in parallel with all "normal" database operations, I'm not that thrilled
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at the prospect of introducing a new mechanism that will block VACUUM.
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Especially not one that's *designed* to hold its lock for a long period
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of time. This will just get us right back into all the operational
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problems that lazy VACUUM was intended to get around. For example, this
|
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one: if transaction A has an insensitive-cursor lock on table T, and a
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VACUUM comes along to vacuum T and blocks waiting for the lock, then
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when subsequent transaction B wants to create an insensitive cursor on T
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it's going to be forced to queue up behind the VACUUM.
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While temp tables may seem like an ugly, low-tech way to support
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insensitive cursors, I think they may have more merit than you realize.
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regards, tom lane
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Date: Fri, 25 Jan 2002 17:05:05 +0100
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To: Tom Lane <tgl@sss.pgh.pa.us>
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cc: Hiroshi Inoue <Inoue@tpf.co.jp>, Bruce Momjian <pgman@candle.pha.pa.us>,
|
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pgsql-general@postgresql.org
|
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Subject: Re: [GENERAL] persistent portals/cursors (between transactions)
|
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References: <200201250319.g0P3Jq022575@candle.pha.pa.us> <23244.1011932544@sss.pgh.pa.us> <3C510D24.8E1FDF7F@tpf.co.jp> <25361.1011971571@sss.pgh.pa.us>
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Tom Lane wrote:
|
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>
|
||||
> Hiroshi Inoue <Inoue@tpf.co.jp> writes:
|
||||
> > Tom Lane wrote:
|
||||
> >> If it's not holding any locks, I can guarantee you it's not insensitive.
|
||||
> >> Consider VACUUM, or even DROP TABLE.
|
||||
>
|
||||
> > It's already possible to keep a lock accross transactions.
|
||||
> > So it would keep an AccessShareLock across transactions.
|
||||
>
|
||||
> AccessShareLock would fend off DROP/ALTER TABLE, but not VACUUM anymore.
|
||||
> We'd need to invent Yet Another lock type that would prevent VACUUM.
|
||||
> Clearly that's perfectly doable.
|
||||
>
|
||||
> But: having just finished a lot of work to ensure that VACUUM could run
|
||||
> in parallel with all "normal" database operations, I'm not that thrilled
|
||||
> at the prospect of introducing a new mechanism that will block VACUUM.
|
||||
> Especially not one that's *designed* to hold its lock for a long period
|
||||
> of time. This will just get us right back into all the operational
|
||||
> problems that lazy VACUUM was intended to get around. For example, this
|
||||
> one: if transaction A has an insensitive-cursor lock on table T, and a
|
||||
> VACUUM comes along to vacuum T and blocks waiting for the lock, then
|
||||
> when subsequent transaction B wants to create an insensitive cursor on T
|
||||
> it's going to be forced to queue up behind the VACUUM.
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Why do you have to lock the whole table when all you want is just one
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set of rows from a set of versions? Am I missing something here?
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When you're talking about in-transaction cursors for the above example,
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why would the cursor need anything more than the transaction A needs
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anyway? And for cross-transaction cursors, why lock the whole table when
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you could use the transaction information from the transaction in which
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the cursor was declared?
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|
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Generally spoken, where's the difference between an insensitive
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persistent cursor and a still running transaction?
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|
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> While temp tables may seem like an ugly, low-tech way to support
|
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> insensitive cursors, I think they may have more merit than you realize.
|
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|
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Obviously, that's the easy way to do it, and lots of other databases
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make use of that already to implement insensitive cursors (see my other
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post). But as the long-term goal should be updateable insensitive
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persistent cursors, I think the temp table solution will get really
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messy.
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From pgsql-general-owner+M19851=candle.pha.pa.us=pgman@postgresql.org Fri Jan 25 11:50:42 2002
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To: Florian Wunderlich <fwunderlich@devbrain.de>
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cc: Hiroshi Inoue <Inoue@tpf.co.jp>, Bruce Momjian <pgman@candle.pha.pa.us>,
|
||||
pgsql-general@postgresql.org
|
||||
Subject: Re: [GENERAL] persistent portals/cursors (between transactions)
|
||||
In-Reply-To: <3C518231.F65DC636@hq.factor3.com>
|
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References: <200201250319.g0P3Jq022575@candle.pha.pa.us> <23244.1011932544@sss.pgh.pa.us> <3C510D24.8E1FDF7F@tpf.co.jp> <25361.1011971571@sss.pgh.pa.us> <3C518231.F65DC636@hq.factor3.com>
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Comments: In-reply-to Florian Wunderlich <fwunderlich@devbrain.de>
|
||||
message dated "Fri, 25 Jan 2002 17:05:05 +0100"
|
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Date: Fri, 25 Jan 2002 11:24:15 -0500
|
||||
Message-ID: <25888.1011975855@sss.pgh.pa.us>
|
||||
From: Tom Lane <tgl@sss.pgh.pa.us>
|
||||
Precedence: bulk
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Sender: pgsql-general-owner@postgresql.org
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Status: OR
|
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|
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Florian Wunderlich <fwunderlich@devbrain.de> writes:
|
||||
> When you're talking about in-transaction cursors for the above example,
|
||||
> why would the cursor need anything more than the transaction A needs
|
||||
> anyway?
|
||||
|
||||
It wouldn't.
|
||||
|
||||
> And for cross-transaction cursors, why lock the whole table when
|
||||
> you could use the transaction information from the transaction in which
|
||||
> the cursor was declared?
|
||||
|
||||
The problem is to keep the rows that are supposed to be still visible to
|
||||
you from disappearing. If other backends think that transaction A is
|
||||
history, they will not think that they need to preserve rows that would
|
||||
have been visible to A, but are not visible to any still-running
|
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transaction.
|
||||
|
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[ ... thinks for awhile ... ] Maybe we could extend the notion of
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"oldest XMIN" a little. Perhaps what each backend should record in the
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PROC array is not just the oldest XMIN visible to its current
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transaction, but the oldest XMIN visible to either its current xact or
|
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any of its open cross-transaction cursors. That together with an
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AccessShareLock on tables referenced by the cursors might work.
|
||||
|
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A drawback of this approach is that opening a cursor and sitting on it
|
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for a long time would effectively defeat VACUUM activity --- it wouldn't
|
||||
be blocked, but it wouldn't be able to reclaim rows either. Anywhere,
|
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not only in the tables actually used by the cursor.
|
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|
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regards, tom lane
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From Inoue@tpf.co.jp Fri Jan 25 11:58:04 2002
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Sat, 26 Jan 2002 01:57:47 +0900 (JST)
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From: "Hiroshi Inoue" <Inoue@tpf.co.jp>
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To: "Tom Lane" <tgl@sss.pgh.pa.us>
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cc: "Bruce Momjian" <pgman@candle.pha.pa.us>,
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"Florian Wunderlich" <fwunderlich@devbrain.de>,
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<pgsql-general@postgresql.org>
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Subject: RE: [GENERAL] persistent portals/cursors (between transactions)
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Date: Sat, 26 Jan 2002 01:57:54 +0900
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> -----Original Message-----
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> From: Tom Lane [mailto:tgl@sss.pgh.pa.us]
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>
|
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> Hiroshi Inoue <Inoue@tpf.co.jp> writes:
|
||||
> > Tom Lane wrote:
|
||||
> >> If it's not holding any locks, I can guarantee you it's not
|
||||
> insensitive.
|
||||
> >> Consider VACUUM, or even DROP TABLE.
|
||||
>
|
||||
> > It's already possible to keep a lock accross transactions.
|
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> > So it would keep an AccessShareLock across transactions.
|
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>
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> AccessShareLock would fend off DROP/ALTER TABLE, but not VACUUM anymore.
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Really ? VACUUM FULL conflicts with AccessShareLock from the
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first. If new vacuum does wrong thing with persistent read-only cursors
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it would do the wrong thing with the current cursors as well.
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Of cource as Vadim mentioned before, HeapTupleSatisfiesVacuum()
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should take the transaction id in which the cursor was opened into
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account.
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regards,
|
||||
Hiroshi Inoue
|
||||
|
||||
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From pgsql-general-owner+M19852=candle.pha.pa.us=pgman@postgresql.org Fri Jan 25 12:04:58 2002
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cc: Hiroshi Inoue <Inoue@tpf.co.jp>, Bruce Momjian <pgman@candle.pha.pa.us>,
|
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pgsql-general@postgresql.org
|
||||
Subject: Re: [GENERAL] persistent portals/cursors (between transactions)
|
||||
References: <200201250319.g0P3Jq022575@candle.pha.pa.us> <23244.1011932544@sss.pgh.pa.us> <3C510D24.8E1FDF7F@tpf.co.jp> <25361.1011971571@sss.pgh.pa.us> <3C518231.F65DC636@hq.factor3.com> <25888.1011975855@sss.pgh.pa.us>
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Status: OR
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|
||||
> > And for cross-transaction cursors, why lock the whole table when
|
||||
> > you could use the transaction information from the transaction in which
|
||||
> > the cursor was declared?
|
||||
>
|
||||
> The problem is to keep the rows that are supposed to be still visible to
|
||||
> you from disappearing. If other backends think that transaction A is
|
||||
> history, they will not think that they need to preserve rows that would
|
||||
> have been visible to A, but are not visible to any still-running
|
||||
> transaction.
|
||||
>
|
||||
> [ ... thinks for awhile ... ] Maybe we could extend the notion of
|
||||
> "oldest XMIN" a little. Perhaps what each backend should record in the
|
||||
> PROC array is not just the oldest XMIN visible to its current
|
||||
> transaction, but the oldest XMIN visible to either its current xact or
|
||||
> any of its open cross-transaction cursors. That together with an
|
||||
> AccessShareLock on tables referenced by the cursors might work.
|
||||
>
|
||||
> A drawback of this approach is that opening a cursor and sitting on it
|
||||
> for a long time would effectively defeat VACUUM activity --- it wouldn't
|
||||
> be blocked, but it wouldn't be able to reclaim rows either. Anywhere,
|
||||
> not only in the tables actually used by the cursor.
|
||||
|
||||
Isn't that exactly what beginning a transaction and keeping it
|
||||
uncommitted for a long time would do too?
|
||||
|
||||
I see the problem - your last sentence - but getting rid of that would
|
||||
mean to not only save an oldest XMIN, but also a reference to all tables
|
||||
that this not-quite-a-xact uses, kind of like a "selective transaction".
|
||||
I doubt that there really are any problems in the real world though, so
|
||||
having a naive implementation first would be fine too.
|
||||
|
||||
So from the vacuum perspective, it looks like more than just one
|
||||
transaction is running per backend, right? Probably I don't understand
|
||||
anything at all, or that's what I suggested way back in my second or
|
||||
third mail. Whatever. Assuming I understood a bit here, a read-write
|
||||
cross-transaction cursor shouldn't be too hard to implement then either.
|
||||
|
||||
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TIP 1: subscribe and unsubscribe commands go to majordomo@postgresql.org
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From pgsql-general-owner+M19855=candle.pha.pa.us=pgman@postgresql.org Fri Jan 25 12:21:10 2002
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Fri, 25 Jan 2002 12:06:09 -0500 (EST)
|
||||
To: "Hiroshi Inoue" <Inoue@tpf.co.jp>
|
||||
cc: "Bruce Momjian" <pgman@candle.pha.pa.us>,
|
||||
"Florian Wunderlich" <fwunderlich@devbrain.de>,
|
||||
pgsql-general@postgresql.org
|
||||
Subject: Re: [GENERAL] persistent portals/cursors (between transactions)
|
||||
In-Reply-To: <EKEJJICOHDIEMGPNIFIJOELEGJAA.Inoue@tpf.co.jp>
|
||||
References: <EKEJJICOHDIEMGPNIFIJOELEGJAA.Inoue@tpf.co.jp>
|
||||
Comments: In-reply-to "Hiroshi Inoue" <Inoue@tpf.co.jp>
|
||||
message dated "Sat, 26 Jan 2002 01:57:54 +0900"
|
||||
Date: Fri, 25 Jan 2002 12:06:08 -0500
|
||||
Message-ID: <26443.1011978368@sss.pgh.pa.us>
|
||||
From: Tom Lane <tgl@sss.pgh.pa.us>
|
||||
Precedence: bulk
|
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Sender: pgsql-general-owner@postgresql.org
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Status: OR
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|
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"Hiroshi Inoue" <Inoue@tpf.co.jp> writes:
|
||||
>> AccessShareLock would fend off DROP/ALTER TABLE, but not VACUUM anymore.
|
||||
|
||||
> Really ? VACUUM FULL conflicts with AccessShareLock from the
|
||||
> first.
|
||||
|
||||
I was speaking of lazy VACUUM, of course.
|
||||
|
||||
> If new vacuum does wrong thing with persistent read-only cursors
|
||||
> it would do the wrong thing with the current cursors as well.
|
||||
|
||||
No, because current cursors don't span transactions.
|
||||
|
||||
> Of cource as Vadim mentioned before, HeapTupleSatisfiesVacuum()
|
||||
> should take the transaction id in which the cursor was opened into
|
||||
> account.
|
||||
|
||||
I haven't read all of that thread yet; maybe Vadim already had the idea
|
||||
I just had of playing games with oldest-XMIN.
|
||||
|
||||
regards, tom lane
|
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|
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From tgl@sss.pgh.pa.us Fri Jan 25 12:07:42 2002
|
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Fri, 25 Jan 2002 12:07:25 -0500 (EST)
|
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To: Florian Wunderlich <fwunderlich@devbrain.de>
|
||||
cc: Hiroshi Inoue <Inoue@tpf.co.jp>, Bruce Momjian <pgman@candle.pha.pa.us>,
|
||||
pgsql-general@postgresql.org
|
||||
Subject: Re: [GENERAL] persistent portals/cursors (between transactions)
|
||||
In-Reply-To: <3C518EC9.FDE6DDC3@hq.factor3.com>
|
||||
References: <200201250319.g0P3Jq022575@candle.pha.pa.us> <23244.1011932544@sss.pgh.pa.us> <3C510D24.8E1FDF7F@tpf.co.jp> <25361.1011971571@sss.pgh.pa.us> <3C518231.F65DC636@hq.factor3.com> <25888.1011975855@sss.pgh.pa.us> <3C518EC9.FDE6DDC3@hq.factor3.com>
|
||||
Comments: In-reply-to Florian Wunderlich <fwunderlich@devbrain.de>
|
||||
message dated "Fri, 25 Jan 2002 17:58:49 +0100"
|
||||
Date: Fri, 25 Jan 2002 12:07:24 -0500
|
||||
Message-ID: <26463.1011978444@sss.pgh.pa.us>
|
||||
From: Tom Lane <tgl@sss.pgh.pa.us>
|
||||
Status: OR
|
||||
|
||||
Florian Wunderlich <fwunderlich@devbrain.de> writes:
|
||||
> Isn't that exactly what beginning a transaction and keeping it
|
||||
> uncommitted for a long time would do too?
|
||||
|
||||
Sure, but then you haven't got a cross-transaction cursor, only a plain
|
||||
cursor.
|
||||
|
||||
regards, tom lane
|
||||
|
||||
From Inoue@tpf.co.jp Fri Jan 25 12:23:39 2002
|
||||
Return-path: <Inoue@tpf.co.jp>
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Sat, 26 Jan 2002 02:23:18 +0900 (JST)
|
||||
From: "Hiroshi Inoue" <Inoue@tpf.co.jp>
|
||||
To: "Florian Wunderlich" <fwunderlich@devbrain.de>
|
||||
cc: "Bruce Momjian" <pgman@candle.pha.pa.us>, "Tom Lane" <tgl@sss.pgh.pa.us>,
|
||||
<pgsql-general@postgresql.org>, "Jan Wieck" <janwieck@yahoo.com>
|
||||
Subject: RE: [GENERAL] persistent portals/cursors (between transactions)
|
||||
Date: Sat, 26 Jan 2002 02:23:26 +0900
|
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Message-ID: <EKEJJICOHDIEMGPNIFIJEELHGJAA.Inoue@tpf.co.jp>
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> -----Original Message-----
|
||||
> From: florian@hq.factor3.com [mailto:florian@hq.factor3.com]On
|
||||
>
|
||||
>
|
||||
> Hiroshi, that's exactly what I need, though I am not sure if we are all
|
||||
> really talking about the same thing.
|
||||
>
|
||||
> In case I misunderstood something: as far as I know, SQL92 defines that
|
||||
> a cursor is by default sensitive, which means that it displays the data
|
||||
> from all comitted transactions at any time. If the data changes, so does
|
||||
> what the cursor returns.
|
||||
|
||||
AFAIK SQL92's default is indeterminate which guarantees nothing
|
||||
about sensitivity. Though we don't have insensitive cursors yet
|
||||
INSENSITIVE cursors are very natural for MVCC and it's not hard
|
||||
to implement. In reality the current cursors see no changes after
|
||||
the cursor was opened other than the ones made by the bakend
|
||||
itself.
|
||||
|
||||
regards,
|
||||
Hiroshi Inoue
|
||||
|
||||
From pgsql-general-owner+M19860=candle.pha.pa.us=pgman@postgresql.org Fri Jan 25 13:16:18 2002
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|
||||
pgsql-general@postgresql.org
|
||||
Subject: Re: [GENERAL] persistent portals/cursors (between transactions)
|
||||
References: <200201250319.g0P3Jq022575@candle.pha.pa.us> <23244.1011932544@sss.pgh.pa.us> <3C510D24.8E1FDF7F@tpf.co.jp> <25361.1011971571@sss.pgh.pa.us> <3C518231.F65DC636@hq.factor3.com> <25888.1011975855@sss.pgh.pa.us> <3C518EC9.FDE6DDC3@hq.factor3.com> <26463.1011978444@sss.pgh.pa.us>
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||||
Tom Lane wrote:
|
||||
> > Isn't that exactly what beginning a transaction and keeping it
|
||||
> > uncommitted for a long time would do too?
|
||||
>
|
||||
> Sure, but then you haven't got a cross-transaction cursor, only a plain
|
||||
> cursor.
|
||||
|
||||
Sorry for being unclear - I wanted to say that this problem obviously
|
||||
already exists, so there's not a new (conceptual) problem here.
|
||||
|
||||
I'm sure you read the second part of my post where I suggested what a
|
||||
possible solution could look like.
|
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