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-From pgsql-general-owner+M19848=candle.pha.pa.us=pgman@postgresql.org Fri Jan 25 10:36:36 2002
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-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: <3C510D24.8E1FDF7F@tpf.co.jp>
-References: <200201250319.g0P3Jq022575@candle.pha.pa.us> <23244.1011932544@sss.pgh.pa.us> <3C510D24.8E1FDF7F@tpf.co.jp>
-Comments: In-reply-to Hiroshi Inoue <Inoue@tpf.co.jp>
- message dated "Fri, 25 Jan 2002 16:45:40 +0900"
-Date: Fri, 25 Jan 2002 10:12:51 -0500
-Message-ID: <25361.1011971571@sss.pgh.pa.us>
-From: Tom Lane <tgl@sss.pgh.pa.us>
-Precedence: bulk
-Sender: pgsql-general-owner@postgresql.org
-Status: OR
-
-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.
-
-While temp tables may seem like an ugly, low-tech way to support
-insensitive cursors, I think they may have more merit than you realize.
-
- regards, tom lane
-
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-From pgsql-general-owner+M19849=candle.pha.pa.us=pgman@postgresql.org Fri Jan 25 11:21:44 2002
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-Date: Fri, 25 Jan 2002 17:05:05 +0100
-From: Florian Wunderlich <fwunderlich@devbrain.de>
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-To: Tom Lane <tgl@sss.pgh.pa.us>
-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)
-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:
->
-> 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.
-
-Why do you have to lock the whole table when all you want is just one
-set of rows from a set of versions? Am I missing something here?
-
-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? 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?
-
-Generally spoken, where's the difference between an insensitive
-persistent cursor and a still running transaction?
-
-> While temp tables may seem like an ugly, low-tech way to support
-> insensitive cursors, I think they may have more merit than you realize.
-
-Obviously, that's the easy way to do it, and lots of other databases
-make use of that already to implement insensitive cursors (see my other
-post). But as the long-term goal should be updateable insensitive
-persistent cursors, I think the temp table solution will get really
-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>
-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>
-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>
-Comments: In-reply-to Florian Wunderlich <fwunderlich@devbrain.de>
- message dated "Fri, 25 Jan 2002 17:05:05 +0100"
-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
-Sender: pgsql-general-owner@postgresql.org
-Status: OR
-
-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
-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.
-
- regards, tom lane
-
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-From Inoue@tpf.co.jp Fri Jan 25 11:58:04 2002
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-From: "Hiroshi Inoue" <Inoue@tpf.co.jp>
-To: "Tom Lane" <tgl@sss.pgh.pa.us>
-cc: "Bruce Momjian" <pgman@candle.pha.pa.us>,
- "Florian Wunderlich" <fwunderlich@devbrain.de>,
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-Subject: RE: [GENERAL] persistent portals/cursors (between transactions)
-Date: Sat, 26 Jan 2002 01:57:54 +0900
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-> -----Original Message-----
-> From: Tom Lane [mailto:tgl@sss.pgh.pa.us]
->
-> 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.
-
-Really ? VACUUM FULL conflicts with AccessShareLock from the
-first. If new vacuum does wrong thing with persistent read-only cursors
-it would do the wrong thing with the current cursors as well.
-Of cource as Vadim mentioned before, HeapTupleSatisfiesVacuum()
-should take the transaction id in which the cursor was opened into
-account.
-
-regards,
-Hiroshi Inoue
-
-
-
-From pgsql-general-owner+M19852=candle.pha.pa.us=pgman@postgresql.org Fri Jan 25 12:04:58 2002
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-Date: Fri, 25 Jan 2002 17:58:49 +0100
-From: Florian Wunderlich <fwunderlich@devbrain.de>
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-To: Tom Lane <tgl@sss.pgh.pa.us>
-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)
-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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-
-> > 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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-From pgsql-general-owner+M19855=candle.pha.pa.us=pgman@postgresql.org Fri Jan 25 12:21:10 2002
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-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>
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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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-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>
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- 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
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-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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-> -----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
-
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-Date: Fri, 25 Jan 2002 18:44:59 +0100
-From: Florian Wunderlich <fwunderlich@devbrain.de>
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-To: Tom Lane <tgl@sss.pgh.pa.us>
-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)
-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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-Status: OR
-
-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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