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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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Fri, 25 Jan 2002 10:12:52 -0500 (EST)
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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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Precedence: bulk
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Sender: pgsql-general-owner@postgresql.org
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Status: OR
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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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---------------------------(end of broadcast)---------------------------
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TIP 4: Don't 'kill -9' the postmaster
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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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Fri, 25 Jan 2002 17:04:25 +0100
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Message-ID: <3C518231.F65DC636@hq.factor3.com>
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Date: Fri, 25 Jan 2002 17:05:05 +0100
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From: Florian Wunderlich <fwunderlich@devbrain.de>
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MIME-Version: 1.0
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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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Precedence: bulk
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Sender: pgsql-general-owner@postgresql.org
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Status: OR
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Tom Lane wrote:
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>
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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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>
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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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>
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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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>
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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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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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Generally spoken, where's the difference between an insensitive
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persistent cursor and a still running transaction?
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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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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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---------------------------(end of broadcast)---------------------------
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TIP 5: Have you checked our extensive FAQ?
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http://www.postgresql.org/users-lounge/docs/faq.html
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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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Fri, 25 Jan 2002 11:24:15 -0500 (EST)
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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>,
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pgsql-general@postgresql.org
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Subject: Re: [GENERAL] persistent portals/cursors (between transactions)
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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>
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message dated "Fri, 25 Jan 2002 17:05:05 +0100"
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Date: Fri, 25 Jan 2002 11:24:15 -0500
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Message-ID: <25888.1011975855@sss.pgh.pa.us>
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From: Tom Lane <tgl@sss.pgh.pa.us>
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Precedence: bulk
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Sender: pgsql-general-owner@postgresql.org
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Status: OR
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Florian Wunderlich <fwunderlich@devbrain.de> writes:
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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?
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It wouldn't.
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> 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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The problem is to keep the rows that are supposed to be still visible to
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you from disappearing. If other backends think that transaction A is
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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
|
|
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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regards, tom lane
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---------------------------(end of broadcast)---------------------------
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TIP 4: Don't 'kill -9' the postmaster
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From Inoue@tpf.co.jp Fri Jan 25 11:58:04 2002
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Return-path: <Inoue@tpf.co.jp>
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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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Status: OR
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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:
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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
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> insensitive.
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> >> Consider VACUUM, or even DROP TABLE.
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>
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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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>
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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,
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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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Return-path: <pgsql-general-owner+M19852=candle.pha.pa.us=pgman@postgresql.org>
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Message-ID: <3C518EC9.FDE6DDC3@hq.factor3.com>
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Date: Fri, 25 Jan 2002 17:58:49 +0100
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From: Florian Wunderlich <fwunderlich@devbrain.de>
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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> <3C518231.F65DC636@hq.factor3.com> <25888.1011975855@sss.pgh.pa.us>
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Sender: pgsql-general-owner@postgresql.org
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Status: OR
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> > 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
|
|
> > the cursor was declared?
|
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>
|
|
> The problem is to keep the rows that are supposed to be still visible to
|
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> 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.
|
|
>
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|
> [ ... 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,
|
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> not only in the tables actually used by the cursor.
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Isn't that exactly what beginning a transaction and keeping it
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uncommitted for a long time would do too?
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I see the problem - your last sentence - but getting rid of that would
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mean to not only save an oldest XMIN, but also a reference to all tables
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that this not-quite-a-xact uses, kind of like a "selective transaction".
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I doubt that there really are any problems in the real world though, so
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having a naive implementation first would be fine too.
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So from the vacuum perspective, it looks like more than just one
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transaction is running per backend, right? Probably I don't understand
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anything at all, or that's what I suggested way back in my second or
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third mail. Whatever. Assuming I understood a bit here, a read-write
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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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Fri, 25 Jan 2002 12:06:09 -0500 (EST)
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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>,
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pgsql-general@postgresql.org
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Subject: Re: [GENERAL] persistent portals/cursors (between transactions)
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In-Reply-To: <EKEJJICOHDIEMGPNIFIJOELEGJAA.Inoue@tpf.co.jp>
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References: <EKEJJICOHDIEMGPNIFIJOELEGJAA.Inoue@tpf.co.jp>
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Comments: In-reply-to "Hiroshi Inoue" <Inoue@tpf.co.jp>
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message dated "Sat, 26 Jan 2002 01:57:54 +0900"
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Date: Fri, 25 Jan 2002 12:06:08 -0500
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Message-ID: <26443.1011978368@sss.pgh.pa.us>
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From: Tom Lane <tgl@sss.pgh.pa.us>
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Precedence: bulk
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Sender: pgsql-general-owner@postgresql.org
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Status: OR
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"Hiroshi Inoue" <Inoue@tpf.co.jp> writes:
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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.
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I was speaking of lazy VACUUM, of course.
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> 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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No, because current cursors don't span transactions.
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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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I haven't read all of that thread yet; maybe Vadim already had the idea
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I just had of playing games with oldest-XMIN.
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regards, tom lane
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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>
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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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In-Reply-To: <3C518EC9.FDE6DDC3@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> <25888.1011975855@sss.pgh.pa.us> <3C518EC9.FDE6DDC3@hq.factor3.com>
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Comments: In-reply-to Florian Wunderlich <fwunderlich@devbrain.de>
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message dated "Fri, 25 Jan 2002 17:58:49 +0100"
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Date: Fri, 25 Jan 2002 12:07:24 -0500
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Message-ID: <26463.1011978444@sss.pgh.pa.us>
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From: Tom Lane <tgl@sss.pgh.pa.us>
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Status: OR
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Florian Wunderlich <fwunderlich@devbrain.de> writes:
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> Isn't that exactly what beginning a transaction and keeping it
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> uncommitted for a long time would do too?
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Sure, but then you haven't got a cross-transaction cursor, only a plain
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cursor.
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regards, tom lane
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From Inoue@tpf.co.jp Fri Jan 25 12:23:39 2002
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Return-path: <Inoue@tpf.co.jp>
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Sat, 26 Jan 2002 02:23:18 +0900 (JST)
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From: "Hiroshi Inoue" <Inoue@tpf.co.jp>
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To: "Florian Wunderlich" <fwunderlich@devbrain.de>
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cc: "Bruce Momjian" <pgman@candle.pha.pa.us>, "Tom Lane" <tgl@sss.pgh.pa.us>,
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<pgsql-general@postgresql.org>, "Jan Wieck" <janwieck@yahoo.com>
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Subject: RE: [GENERAL] persistent portals/cursors (between transactions)
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Date: Sat, 26 Jan 2002 02:23:26 +0900
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Message-ID: <EKEJJICOHDIEMGPNIFIJEELHGJAA.Inoue@tpf.co.jp>
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Status: OR
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> -----Original Message-----
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> From: florian@hq.factor3.com [mailto:florian@hq.factor3.com]On
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>
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>
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> Hiroshi, that's exactly what I need, though I am not sure if we are all
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> really talking about the same thing.
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>
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> In case I misunderstood something: as far as I know, SQL92 defines that
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> a cursor is by default sensitive, which means that it displays the data
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> from all comitted transactions at any time. If the data changes, so does
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> what the cursor returns.
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AFAIK SQL92's default is indeterminate which guarantees nothing
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about sensitivity. Though we don't have insensitive cursors yet
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INSENSITIVE cursors are very natural for MVCC and it's not hard
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to implement. In reality the current cursors see no changes after
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the cursor was opened other than the ones made by the bakend
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itself.
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regards,
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Hiroshi Inoue
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From pgsql-general-owner+M19860=candle.pha.pa.us=pgman@postgresql.org Fri Jan 25 13:16:18 2002
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Message-ID: <3C51999B.260171D6@hq.factor3.com>
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Date: Fri, 25 Jan 2002 18:44:59 +0100
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From: Florian Wunderlich <fwunderlich@devbrain.de>
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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> <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
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|
Tom Lane wrote:
|
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> > Isn't that exactly what beginning a transaction and keeping it
|
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> > uncommitted for a long time would do too?
|
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>
|
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> Sure, but then you haven't got a cross-transaction cursor, only a plain
|
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> cursor.
|
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Sorry for being unclear - I wanted to say that this problem obviously
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already exists, so there's not a new (conceptual) problem here.
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I'm sure you read the second part of my post where I suggested what a
|
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possible solution could look like.
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