10 weeks to feature freeze (Pending Work)

jd@commandprompt.com

about 19 years ago

In reply to: Joshua D. Drake (#1)

Re: Updateable cursors

FAST PostgreSQL wrote:

We are trying to develop the updateable cursors functionality into
Postgresql. I have given below details of the design and also issues we are
facing. Looking forward to the advice on how to proceed with these issues.

Rgds,
Arul Shaji

Would this be something that you would hope to submit for 8.3?

Joshua D. Drake

1. Introduction
--------------
This is a combined proposal and design document for adding updatable
(insensitive) cursor capability to the PostgreSQL database.
There have already been a couple of previous proposals since 2003 for
implementing this feature so there appears to be community interest in doing
so. This will enable the following constructs to be processed:

UPDATE <table_name> SET value_list WHERE CURRENT OF <cursor_name>
DELETE FROM <table_name> WHERE CURRENT OF <cursor_name>

This has the effect of users being able to update or delete specific rows of
a table, as defined by the row currently fetched into the cursor.

2. Overall Conceptual Design
-----------------------------
The design is considered from the viewpoint of progression of a command
through the various stages of processing, from changes to the file ï¿½gram.yï¿½
to implement the actual grammar changes, through to changes in the Executor
portion of the database architecture.

2.1 Changes to the Grammar
------------------------------
The following changes will be done to the PostgreSQL grammar:

UPDATE statement has the option ï¿½WHERE CURRENT OF <cursor_name>ï¿½ added
DELETE statement has the option ï¿½WHERE CURRENT OF <cursor_name>ï¿½ added

The cursor_name data is held in the UpdateStmt and DeleteStmt structures and
contains just the name of the cursor.

The pl/pgsql grammar changes in the same manner.

The word CURRENT will be added to the ScanKeywords array in keywords.c.

2.2 Changes to Affected Data Structures
------------------------------------------
The following data structures are affected by this change:

Portal structure, QueryDesc structure, the UpdateStmt and DeleteStmt
structures

The Portal will contain a list of structures of relation ids and tuple ids
relating to the tuple held in the QueryDesc structure. There will be one
entry in the relation and tuple id list for each entry in the relation-list
of the statement below:

DECLARE <cursor_name> [WITH HOLD] SELECT FOR UPDATE OF <relation-list>

The QueryDesc structure will contain the relation id and the tuple id
relating to the tuple obtained via the FETCH command so that it can be
propagated back to the Portal for storage in the list described above.

The UpdateStmt and DeleteStmt structures have the cursor name added so that
the information is available for use in obtaining the portal structure
related to the cursor previously opened via the DECLARE CURSOR request.

2.3 Changes to the SQL Parser
------------------------------------
At present, although the FOR UPDATE clause of the DECLARE CURSOR command has
been present in the grammar, it causes an error message later in the
processing since cursors are currently not updatable. This now needs to
change. The ï¿½FOR UPDATEï¿½ clause has to be valid, but not the ï¿½FOR SHAREï¿½
clause.

The relation names that follow the ï¿½FOR UPDATEï¿½ clause will be added to the
rtable in the Query structure and identified by means of the rowMarks array.
In the case of an updatable cursor the FOR SHARE option is not allowed
therefore all entries in the rtable that are identified by the rowMarks array
must relate to tables that are FOR UPDATE.

In the UPDATE or DELETE statements the ï¿½WHERE CURRENT OF <cursor_name>ï¿½
clause results in the cursor name being placed in the UpdateStmt or
DeleteStmt structure. During the processing of the functions -
transformDeleteStmt() and transformUpdateStmt() - the cursor name is used to
obtain a pointer to the related Portal structure and the tuple affected by
the current UPDATE or DELETE statement is extracted from the Portal, where it
has been placed as the result of a previous FETCH request. At this point all
the information for the UPDATE or DELETE statement is available so the
statements can be transformed into standard UPDATE or DELETE statements and
sent for re-write/planning/execution as usual.

2.4 Changes to the Optimizer
------------------------------
There is a need to add a TidScan node to planning UPDATE / DELETE statements
where the statements are ï¿½UPDATE / DELETE at positionï¿½. This is to enable the
tuple ids of the tuples in the tables relating to the query to be obtained.
There will need to be a new mechanism to achieve this, as at present, a Tid
scan is done only if there is a standard WHERE condition on update or delete
statements to provide Tid qualifier data.

2.5 Changes to the Executor
-------------------------------
There are various options that have been considered for this part of the
enhancement. These are described in the sections below.

We would like to hear opinions on which option is the best way to go or if
none of these is acceptable, any alternate ideas ?

Option 1 MVCC Via Continuous Searching of Database

The Executor is to be changed in the following ways:
1) When the FETCH statement is executed the id of the resulting tuple is
extracted and passed back to the Portal structure to be saved to indicate the
cursor is currently positioned on a tuple.
2) When the UPDATE or DELETE request is executed the tuple id previously
FETCHed is held in the QueryDesc structure so that it can be compared with
the tuple ids returned from the TidScan node processed prior to the actual
UPDATE / DELETE node in the plan. This enables a decision to be made as to
whether the tuple held in the cursor is visible to the UPDATE / DELETE
request according to the rules of concurrency. The result is that, at the
cost of repeatedly searching the database at each UPDATE / DELETE command,
the hash table is no longer required.
This approach has the advantage that there is no hash table held in memory or
on disk so it will not be memory intensive but will be processing intensive.

This is a good ï¿½one-offï¿½ solution to the problem and, taken in isolation is
probably the best approach. However, if one considers the method(s) used in
other areas of PostgreSQL, it is probably not the best solution. This option
will probably not be used further.

Option 2 MVCC via New Snapshot

The executor can be changed by adding a new kind of snapshot that is
specifically used for identifying if a given tuple, retrieved from the
database during an update or delete statement should be visible during the
current transaction.

This approach requires a new kind of snapshot (this idea was used by Gavin
for a previous updatable cursor patch but objections were raised.)

Option 3 MVCC Via Hash Table in Memory

The executor can be changed by saving into a hash table and comparing each
tuple in the cursor with that set to check if the tuple should be visible.
This approach has the advantage that it will be quick. It has the
disadvantage that, since the hash table will contain all the tuples of the
table being checked that it may use all local memory for a large table.

Option 4 MVCC Via Hash Table on Disk

When the UPDATE or DELETE request is executed the first time the Tid scan
database retrieval will be done first. At this time the tuple id of each row
in the table to be updated by the request will be available in the executor.
These tuple ids need to be stored in a hash table that is stored to disk, as,
if the table is large there could be a huge number of tuple ids. This data is
then available for comparison with the individual tuple to be updated or
deleted to check if it should be processed. The hash table will exist for the
duration of the transaction, from BEGIN to END (or ABORT).

The hash table is then used to identify if the tuple should be visible during
the current transaction. If the tuple should be visible then the update or
delete proceeds as usual.

This approach has the advantage that it will use little memory but will be
relatively slow as the data has to be accessed from disk.

Option 5 Store Tuple Id in Snapshot.

The Snapshot structure can be changed to include the tuple id. This enables
the current state of the tuple to be identified with respect to the current
transaction.
The tuple id, as identified in the cursor at the point where the
DELETE/UPDATE statement is being processed, can use the snapshot to identify
if the tuple should be visible in the context of the current transaction.

2.6 Changes to the Catalog
----------------------------
The Catalog needs to reflect changes introduced by the updatable cursor
implementation. A boolean attribute ï¿½is_for_updateï¿½ is to be added to the
pg_cursors implementation. It will define that the cursor is for update
(value is FALSE) or for share (value is TRUE, the default value).

3 Design Assumptions
----------------------------
The following design assumptions are made:

As PostgreSQL8.2 does not support the SENSITIVE cursor option the tuples
contained in a cursor can never be updated so these tuples will always appear
in their ï¿½originalï¿½ form as at the start of the transaction. This is in
breach of the SQL2003 Standard as described in 5WD-02-Foundation-2003-09.pdf,
p 810. The standard requires the updatable cursor to be declared as sensitive.

With respect to nested transactions ï¿½ In PostgreSQL nested transactions are
implemented by defining ï¿½save pointsï¿½ via the keyword SAVEPOINT. A ï¿½ROLLBACK
TO SAVEPOINTï¿½ rolls back the database contents to the last savepoint in this
transaction or the begin statement, whichever is closer.

It is assumed that the FETCH statement is used to return only a single row
into the cursor with each command when the cursor is updatable.

According to the SQL2003 Standard Update and Delete statements may contain
only a single base table.

The DECLARE CURSOR statement is supposed to use column level locking, but
PostgreSQL supports only row level locking. The result of this is that the
column list that the standard requires ï¿½DECLARE <cursor_name> SELECT ï¿½ FOR
UPDATE OF column-listï¿½ becomes a relation (table) list.

This is an email from Fujitsu Australia Software Technology Pty Ltd, ABN 27 003 693 481. It is confidential to the ordinary user of the email address to which it was addressed and may contain copyright and/or legally privileged information. No one else may read, print, store, copy or forward all or any of it or its attachments. If you receive this email in error, please return to sender. Thank you.

If you do not wish to receive commercial email messages from Fujitsu Australia Software Technology Pty Ltd, please email unsubscribe@fast.fujitsu.com.au

---------------------------(end of broadcast)---------------------------
TIP 9: In versions below 8.0, the planner will ignore your desire to
choose an index scan if your joining column's datatypes do not
match

Donate to the PostgreSQL Project: http://www.postgresql.org/about/donate
PostgreSQL Replication: http://www.commandprompt.com/products/

Import Notes

Reply to msg id not found: 5537.10001169527168.fast.fujitsu.com.au@MHS

jd@commandprompt.com

about 19 years ago

In reply to: Joshua D. Drake (#1)

Re: Updateable cursors

FAST PostgreSQL wrote:

On Tue, 23 Jan 2007 15:48, Joshua D. Drake wrote:

FAST PostgreSQL wrote:

We are trying to develop the updateable cursors functionality into
Postgresql. I have given below details of the design and also issues we
are facing. Looking forward to the advice on how to proceed with these
issues.

Rgds,
Arul Shaji

Would this be something that you would hope to submit for 8.3?

Yes definitely. If we can finish it before the feature freeze of course.

Great! I will put it on my, "Remember to bug Arul" list :)

Sincerely,

Joshua D. Drake

Rgds,
Arul Shaji

Joshua D. Drake

1. Introduction
--------------
This is a combined proposal and design document for adding updatable
(insensitive) cursor capability to the PostgreSQL database.
There have already been a couple of previous proposals since 2003 for
implementing this feature so there appears to be community interest in
doing so. This will enable the following constructs to be processed:

UPDATE <table_name> SET value_list WHERE CURRENT OF <cursor_name>
DELETE FROM <table_name> WHERE CURRENT OF <cursor_name>

This has the effect of users being able to update or delete specific rows
of a table, as defined by the row currently fetched into the cursor.

2. Overall Conceptual Design
-----------------------------
The design is considered from the viewpoint of progression of a command
through the various stages of processing, from changes to the file
?gram.y? to implement the actual grammar changes, through to changes in
the Executor portion of the database architecture.

2.1 Changes to the Grammar
------------------------------
The following changes will be done to the PostgreSQL grammar:

UPDATE statement has the option ?WHERE CURRENT OF <cursor_name>? added
DELETE statement has the option ?WHERE CURRENT OF <cursor_name>? added

The cursor_name data is held in the UpdateStmt and DeleteStmt structures
and contains just the name of the cursor.

The pl/pgsql grammar changes in the same manner.

The word CURRENT will be added to the ScanKeywords array in keywords.c.

2.2 Changes to Affected Data Structures
------------------------------------------
The following data structures are affected by this change:

Portal structure, QueryDesc structure, the UpdateStmt and DeleteStmt
structures

The Portal will contain a list of structures of relation ids and tuple
ids relating to the tuple held in the QueryDesc structure. There will be
one entry in the relation and tuple id list for each entry in the
relation-list of the statement below:

DECLARE <cursor_name> [WITH HOLD] SELECT FOR UPDATE OF <relation-list>

The QueryDesc structure will contain the relation id and the tuple id
relating to the tuple obtained via the FETCH command so that it can be
propagated back to the Portal for storage in the list described above.

The UpdateStmt and DeleteStmt structures have the cursor name added so
that the information is available for use in obtaining the portal
structure related to the cursor previously opened via the DECLARE CURSOR
request.

2.3 Changes to the SQL Parser
------------------------------------
At present, although the FOR UPDATE clause of the DECLARE CURSOR command
has been present in the grammar, it causes an error message later in the
processing since cursors are currently not updatable. This now needs to
change. The ?FOR UPDATE? clause has to be valid, but not the ?FOR SHARE?
clause.

The relation names that follow the ?FOR UPDATE? clause will be added to
the rtable in the Query structure and identified by means of the rowMarks
array. In the case of an updatable cursor the FOR SHARE option is not
allowed therefore all entries in the rtable that are identified by the
rowMarks array must relate to tables that are FOR UPDATE.

In the UPDATE or DELETE statements the ?WHERE CURRENT OF <cursor_name>?
clause results in the cursor name being placed in the UpdateStmt or
DeleteStmt structure. During the processing of the functions -
transformDeleteStmt() and transformUpdateStmt() - the cursor name is used
to obtain a pointer to the related Portal structure and the tuple
affected by the current UPDATE or DELETE statement is extracted from the
Portal, where it has been placed as the result of a previous FETCH
request. At this point all the information for the UPDATE or DELETE
statement is available so the statements can be transformed into standard
UPDATE or DELETE statements and sent for re-write/planning/execution as
usual.

2.4 Changes to the Optimizer
------------------------------
There is a need to add a TidScan node to planning UPDATE / DELETE
statements where the statements are ?UPDATE / DELETE at position?. This
is to enable the tuple ids of the tuples in the tables relating to the
query to be obtained. There will need to be a new mechanism to achieve
this, as at present, a Tid scan is done only if there is a standard WHERE
condition on update or delete statements to provide Tid qualifier data.

2.5 Changes to the Executor
-------------------------------
There are various options that have been considered for this part of the
enhancement. These are described in the sections below.

We would like to hear opinions on which option is the best way to go or
if none of these is acceptable, any alternate ideas ?

Option 1 MVCC Via Continuous Searching of Database

The Executor is to be changed in the following ways:
1) When the FETCH statement is executed the id of the resulting tuple is
extracted and passed back to the Portal structure to be saved to indicate
the cursor is currently positioned on a tuple.
2) When the UPDATE or DELETE request is executed the tuple id previously
FETCHed is held in the QueryDesc structure so that it can be compared
with the tuple ids returned from the TidScan node processed prior to the
actual UPDATE / DELETE node in the plan. This enables a decision to be
made as to whether the tuple held in the cursor is visible to the UPDATE
/ DELETE request according to the rules of concurrency. The result is
that, at the cost of repeatedly searching the database at each UPDATE /
DELETE command, the hash table is no longer required.
This approach has the advantage that there is no hash table held in
memory or on disk so it will not be memory intensive but will be
processing intensive.

This is a good ?one-off? solution to the problem and, taken in isolation
is probably the best approach. However, if one considers the method(s)
used in other areas of PostgreSQL, it is probably not the best solution.
This option will probably not be used further.

Option 2 MVCC via New Snapshot

The executor can be changed by adding a new kind of snapshot that is
specifically used for identifying if a given tuple, retrieved from the
database during an update or delete statement should be visible during
the current transaction.

This approach requires a new kind of snapshot (this idea was used by
Gavin for a previous updatable cursor patch but objections were raised.)

Option 3 MVCC Via Hash Table in Memory

The executor can be changed by saving into a hash table and comparing
each tuple in the cursor with that set to check if the tuple should be
visible. This approach has the advantage that it will be quick. It has
the disadvantage that, since the hash table will contain all the tuples
of the table being checked that it may use all local memory for a large
table.

Option 4 MVCC Via Hash Table on Disk

When the UPDATE or DELETE request is executed the first time the Tid scan
database retrieval will be done first. At this time the tuple id of each
row in the table to be updated by the request will be available in the
executor. These tuple ids need to be stored in a hash table that is
stored to disk, as, if the table is large there could be a huge number of
tuple ids. This data is then available for comparison with the individual
tuple to be updated or deleted to check if it should be processed. The
hash table will exist for the duration of the transaction, from BEGIN to
END (or ABORT).

The hash table is then used to identify if the tuple should be visible
during the current transaction. If the tuple should be visible then the
update or delete proceeds as usual.

This approach has the advantage that it will use little memory but will
be relatively slow as the data has to be accessed from disk.

Option 5 Store Tuple Id in Snapshot.

The Snapshot structure can be changed to include the tuple id. This
enables the current state of the tuple to be identified with respect to
the current transaction.
The tuple id, as identified in the cursor at the point where the
DELETE/UPDATE statement is being processed, can use the snapshot to
identify if the tuple should be visible in the context of the current
transaction.

2.6 Changes to the Catalog
----------------------------
The Catalog needs to reflect changes introduced by the updatable cursor
implementation. A boolean attribute ?is_for_update? is to be added to the
pg_cursors implementation. It will define that the cursor is for update
(value is FALSE) or for share (value is TRUE, the default value).

3 Design Assumptions
----------------------------
The following design assumptions are made:

As PostgreSQL8.2 does not support the SENSITIVE cursor option the tuples
contained in a cursor can never be updated so these tuples will always
appear in their ?original? form as at the start of the transaction. This
is in breach of the SQL2003 Standard as described in
5WD-02-Foundation-2003-09.pdf, p 810. The standard requires the updatable
cursor to be declared as sensitive.

With respect to nested transactions ? In PostgreSQL nested transactions
are implemented by defining ?save points? via the keyword SAVEPOINT. A
?ROLLBACK TO SAVEPOINT? rolls back the database contents to the last
savepoint in this transaction or the begin statement, whichever is
closer.

It is assumed that the FETCH statement is used to return only a single
row into the cursor with each command when the cursor is updatable.

According to the SQL2003 Standard Update and Delete statements may
contain only a single base table.

The DECLARE CURSOR statement is supposed to use column level locking, but
PostgreSQL supports only row level locking. The result of this is that
the column list that the standard requires ?DECLARE <cursor_name> SELECT
? FOR UPDATE OF column-list? becomes a relation (table) list.

This is an email from Fujitsu Australia Software Technology Pty Ltd, ABN
27 003 693 481. It is confidential to the ordinary user of the email
address to which it was addressed and may contain copyright and/or
legally privileged information. No one else may read, print, store, copy
or forward all or any of it or its attachments. If you receive this email
in error, please return to sender. Thank you.

If you do not wish to receive commercial email messages from Fujitsu
Australia Software Technology Pty Ltd, please email
unsubscribe@fast.fujitsu.com.au

---------------------------(end of broadcast)---------------------------
TIP 9: In versions below 8.0, the planner will ignore your desire to
choose an index scan if your joining column's datatypes do not
match

This is an email from Fujitsu Australia Software Technology Pty Ltd, ABN 27 003 693 481. It is confidential to the ordinary user of the email address to which it was addressed and may contain copyright and/or legally privileged information. No one else may read, print, store, copy or forward all or any of it or its attachments. If you receive this email in error, please return to sender. Thank you.

If you do not wish to receive commercial email messages from Fujitsu Australia Software Technology Pty Ltd, please email unsubscribe@fast.fujitsu.com.au

Donate to the PostgreSQL Project: http://www.postgresql.org/about/donate
PostgreSQL Replication: http://www.commandprompt.com/products/

Import Notes

Reply to msg id not found: 5537.10011169527796.fast.fujitsu.com.au@MHS

Lukas Kahwe Smith

smith@pooteeweet.org

about 19 years ago

In reply to: Joshua D. Drake (#8)

Re: Updateable cursors

Joshua D. Drake wrote:

Great! I will put it on my, "Remember to bug Arul" list :)

Hey Joshua,

could you put this stuff here:
http://developer.postgresql.org/index.php/Todo:WishlistFor83

I will try to find some time during this week (likely on the weekend) to
also try and figure out if these items are real and if the people still
think they can do them for 8.3 .. your additions would be most helpful.

regards,
Lukas

#10

jd@commandprompt.com

about 19 years ago

In reply to: Lukas Kahwe Smith (#9)

Re: Updateable cursors

Lukas Kahwe Smith wrote:

Joshua D. Drake wrote:

Great! I will put it on my, "Remember to bug Arul" list :)

Hey Joshua,

could you put this stuff here:
http://developer.postgresql.org/index.php/Todo:WishlistFor83

Sure if you bother to unlock the page for me ;)

I will try to find some time during this week (likely on the weekend) to
also try and figure out if these items are real and if the people still
think they can do them for 8.3 .. your additions would be most helpful.

regards,
Lukas

---------------------------(end of broadcast)---------------------------
TIP 3: Have you checked our extensive FAQ?

http://www.postgresql.org/docs/faq

Donate to the PostgreSQL Project: http://www.postgresql.org/about/donate
PostgreSQL Replication: http://www.commandprompt.com/products/

#11

Heikki Linnakangas

heikki.linnakangas@enterprisedb.com

about 19 years ago

In reply to: Joshua D. Drake (#1)

Re: 10 weeks to feature freeze (Pending Work)

Joshua D. Drake wrote:

Or so... :)

Thought I would do a poll of what is happening in the world for 8.3. I have:

Alvaro Herrera: Autovacuum improvements (maintenance window etc..)
Gavin Sherry: Bitmap Indexes (on disk), possible basic Window functions

Gavin: how's it going with the bitmap indexes? I could work on it as
well, but I don't want to step on your toes.

Heikki Linnakangas: Working on Vacuum for Bitmap Indexes?

Yeah, that's the plan.

Also:
* Grouped Index Tuples (http://community.enterprisedb.com/git/). I don't
know how to proceed with this, but it's a feature I'd like to get in
8.3. Suggestions, anyone? I haven't received much comments on the design
or code...

* vacuum enhancements, not sure what exactly..

* Plan invalidation, possibly. Tom had plans on this as well.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com

#12

Pavan Deolasee

pavan.deolasee@gmail.com

about 19 years ago

In reply to: Joshua D. Drake (#1)

Re: 10 weeks to feature freeze (Pending Work)

On 1/23/07, Joshua D. Drake <jd@commandprompt.com> wrote:

Or so... :)

I am sure there are more, the ones with question marks are unknowns but
heard of in the ether somewhere. Any additions or confirmations?

I have the first phase of Frequent Update Optimizations (HOT) patch ready.
But I held it back because of the concerns that its too complex. It has
shown decent performance gains on pgbench and DBT2 tests though.

I am splitting the patch into smaller pieces for ease of review and would
submit those soon for comments.

Thanks,
Pavan

EnterpriseDB http://www.enterprisedb.com

#13

Magnus Hagander

magnus@hagander.net

about 19 years ago

In reply to: Stephen Frost (#6)

Re: 10 weeks to feature freeze (Pending Work)

On Mon, Jan 22, 2007 at 09:14:01PM -0500, Stephen Frost wrote:

* Joshua D. Drake (jd@commandprompt.com) wrote:

Thought I would do a poll of what is happening in the world for 8.3. I have:

Another thing which was mentioned previously which I'd really like to
see happen (and was discussed on the list...) is replacing the Kerberos
support with GSSAPI support and adding support for SSPI. Don't recall
who had said they were looking into working on it though..

That's Henry B. Hotz. He's done some work on it, and I have some stuff
to comment on sitting in my mailbox that I haven't had time to look at
yet. But I'm going to try to do that soon so he can continue.

//Magnus

#14

simon@2ndQuadrant.com

about 19 years ago

In reply to: Joshua D. Drake (#1)

Re: Updateable cursors

On Wed, 2007-01-24 at 02:42 +1100, FAST PostgreSQL wrote:

In the UPDATE or DELETE statements the ‘WHERE CURRENT OF <cursor_name>’
clause results in the cursor name being placed in the UpdateStmt or
DeleteStmt structure. During the processing of the functions -
transformDeleteStmt() and transformUpdateStmt() - the cursor name is used to
obtain a pointer to the related Portal structure

To support prepared statements we'd need to do this name lookup just
once, so that the Update/Delete stmt can record which Portal to look at
for the current tuple.

and the tuple affected by
the current UPDATE or DELETE statement is extracted from the Portal, where it
has been placed as the result of a previous FETCH request. At this point all
the information for the UPDATE or DELETE statement is available so the
statements can be transformed into standard UPDATE or DELETE statements and
sent for re-write/planning/execution as usual.

2.5 Changes to the Executor
-------------------------------
There are various options that have been considered for this part of the
enhancement. These are described in the sections below.

Option 1 MVCC Via Continuous Searching of Database

The Executor is to be changed in the following ways:
1) When the FETCH statement is executed the id of the resulting tuple is
extracted and passed back to the Portal structure to be saved to indicate the
cursor is currently positioned on a tuple.
2) When the UPDATE or DELETE request is executed the tuple id previously
FETCHed is held in the QueryDesc structure so that it can be compared with
the tuple ids returned from the TidScan node processed prior to the actual
UPDATE / DELETE node in the plan. This enables a decision to be made as to
whether the tuple held in the cursor is visible to the UPDATE / DELETE
request according to the rules of concurrency. The result is that, at the
cost of repeatedly searching the database at each UPDATE / DELETE command,
the hash table is no longer required.
This approach has the advantage that there is no hash table held in memory or
on disk so it will not be memory intensive but will be processing intensive.

Do you have a specific example that would cause problems? It's much
easier to give examples that might cause problems and discuss those.

AFAICS in the straightforward case the Fetch will only return rows it
can see so update/delete should have no problems, iff the update/delete
is using a same or later snapshot than the cursor. I can see potential
problems with scrollable cursors.

So I'm not sure why there's a big need for any of the 5 options, yet.

--
Simon Riggs
EnterpriseDB http://www.enterprisedb.com

Import Notes

Reply to msg id not found: 5537.10001169527168.fast.fujitsu.com.au@MHS

#15

Lukas Kahwe Smith

smith@pooteeweet.org

about 19 years ago

In reply to: Joshua D. Drake (#10)

Re: Updateable cursors

Joshua D. Drake wrote:

Lukas Kahwe Smith wrote:

Joshua D. Drake wrote:

Great! I will put it on my, "Remember to bug Arul" list :)

Hey Joshua,

could you put this stuff here:
http://developer.postgresql.org/index.php/Todo:WishlistFor83

Sure if you bother to unlock the page for me ;)

hmm .. i am not aware of having a lock. i dont know mediawiki all that
well, but clicking around i could not find anything. IIRC someone else
also had issues editing pages on the wiki.

regards,
Lukas

#16

Iannsp

iannsp@gmail.com

about 19 years ago

In reply to: Pavan Deolasee (#12)

About PostgreSQL certification

Hello,
I did like to know what you think about the postgresql certifications
provided for

PostgreSQL CE
http://www.sraoss.co.jp/postgresql-ce/news_en.html

CertFirst
http://www.certfirst.com/postgreSql.htm

My question is about the validate of this certification for the clients.
Make difference to be certified?

thanks for advanced.

Ivo Nascimento.

#17

Teodor Sigaev

teodor@sigaev.ru

about 19 years ago

In reply to: Joshua D. Drake (#1)

Re: 10 weeks to feature freeze (Pending Work)

I would like to suggest patches for OR-clause optimization and using index for
searching NULLs.

--
Teodor Sigaev E-mail: teodor@sigaev.ru
WWW: http://www.sigaev.ru/

#18

Tom Lane

tgl@sss.pgh.pa.us

about 19 years ago

In reply to: Simon Riggs (#14)

Re: Updateable cursors

"Simon Riggs" <simon@2ndquadrant.com> writes:

On Wed, 2007-01-24 at 02:42 +1100, FAST PostgreSQL wrote:

In the UPDATE or DELETE statements the ‘WHERE CURRENT OF <cursor_name>’
clause results in the cursor name being placed in the UpdateStmt or
DeleteStmt structure. During the processing of the functions -
transformDeleteStmt() and transformUpdateStmt() - the cursor name is used to
obtain a pointer to the related Portal structure

To support prepared statements we'd need to do this name lookup just
once, so that the Update/Delete stmt can record which Portal to look at
for the current tuple.

This really isn't gonna work, because it assumes that the tuple that is
"current" at the instant of parsing is still going to be "current" at
execution time.

regards, tom lane

#19

simon@2ndQuadrant.com

about 19 years ago

In reply to: Tom Lane (#18)

Re: Updateable cursors

On Tue, 2007-01-23 at 09:55 -0500, Tom Lane wrote:

"Simon Riggs" <simon@2ndquadrant.com> writes:

On Wed, 2007-01-24 at 02:42 +1100, FAST PostgreSQL wrote:

In the UPDATE or DELETE statements the ‘WHERE CURRENT OF <cursor_name>’
clause results in the cursor name being placed in the UpdateStmt or
DeleteStmt structure. During the processing of the functions -
transformDeleteStmt() and transformUpdateStmt() - the cursor name is used to
obtain a pointer to the related Portal structure

To support prepared statements we'd need to do this name lookup just
once, so that the Update/Delete stmt can record which Portal to look at
for the current tuple.

This really isn't gonna work, because it assumes that the tuple that is
"current" at the instant of parsing is still going to be "current" at
execution time.

Of course thats true, but you've misread my comment.

The portal with the cursor in will not change, no matter how many times
we execute WHERE CURRENT OF in another portal. The OP suggested putting
the current tuple pointer onto the portal data, so this will work.

--
Simon Riggs
EnterpriseDB http://www.enterprisedb.com

#20

Tom Lane

tgl@sss.pgh.pa.us

about 19 years ago

In reply to: Simon Riggs (#19)

Re: Updateable cursors

"Simon Riggs" <simon@2ndquadrant.com> writes:

On Tue, 2007-01-23 at 09:55 -0500, Tom Lane wrote:

This really isn't gonna work, because it assumes that the tuple that is
"current" at the instant of parsing is still going to be "current" at
execution time.

Of course thats true, but you've misread my comment.

The portal with the cursor in will not change, no matter how many times
we execute WHERE CURRENT OF in another portal.

Really? The cursor portal will cease to exist as soon as the
transaction ends, but the prepared plan won't. A reasonable person
would expect that WHERE CURRENT OF will parse into a plan that just
stores the cursor name, and looks up the cursor at execution time.

The OP suggested putting
the current tuple pointer onto the portal data, so this will work.

No, as I read his message he was suggesting pulling data out of the
cursor portal at plan time so that no downstream (executor) changes
would be needed. That is certainly never going to be workable.

regards, tom lane

#21

FAST PostgreSQL

fastpgs@fast.fujitsu.com.au

about 19 years ago

In reply to: Stephen Frost (#6)

#22

FAST PostgreSQL

fastpgs@fast.fujitsu.com.au

about 19 years ago

In reply to: Joshua D. Drake (#7)

#23

jd@commandprompt.com

about 19 years ago

In reply to: Pavan Deolasee (#12)

#24

simon@2ndQuadrant.com

about 19 years ago

In reply to: Tom Lane (#20)

#25

Richard Troy

rtroy@ScienceTools.com

about 19 years ago

In reply to: FAST PostgreSQL (#21)

#26

Merlin Moncure

mmoncure@gmail.com

about 19 years ago

In reply to: Joshua D. Drake (#1)

#27

jd@commandprompt.com

about 19 years ago

In reply to: Iannsp (#16)

#28

David Fetter

david@fetter.org

about 19 years ago

In reply to: Iannsp (#16)

#29

jesus@omniti.com

about 19 years ago

In reply to: David Fetter (#28)

#30

David Fetter

david@fetter.org

about 19 years ago

In reply to: Theo Schlossnagle (#29)

#31

Mark Kirkwood

mark.kirkwood@catalyst.net.nz

about 19 years ago

In reply to: Theo Schlossnagle (#29)

#32

jesus@omniti.com

about 19 years ago

In reply to: Mark Kirkwood (#31)

#33

jd@commandprompt.com

about 19 years ago

In reply to: Theo Schlossnagle (#29)

#34

jd@commandprompt.com

about 19 years ago

In reply to: Theo Schlossnagle (#32)

#35

jesus@omniti.com

about 19 years ago

In reply to: Joshua D. Drake (#34)

#36

David Fetter

david@fetter.org

about 19 years ago

In reply to: Theo Schlossnagle (#35)

#37

jd@commandprompt.com

about 19 years ago

In reply to: Theo Schlossnagle (#35)

#38

Mark Kirkwood

mark.kirkwood@catalyst.net.nz

about 19 years ago

In reply to: Theo Schlossnagle (#32)

#39

jd@commandprompt.com

about 19 years ago

In reply to: Mark Kirkwood (#38)

#40

Mark Kirkwood

mark.kirkwood@catalyst.net.nz

about 19 years ago

In reply to: Joshua D. Drake (#39)

#41

jd@commandprompt.com

about 19 years ago

In reply to: Mark Kirkwood (#40)

#42

John Bartlett

johnb@fast.fujitsu.com.au

about 19 years ago

In reply to: Richard Troy (#25)

#43

John Bartlett

johnb@fast.fujitsu.com.au

about 19 years ago

In reply to: Simon Riggs (#14)

#44

Jim Nasby

Jim.Nasby@BlueTreble.com

about 19 years ago

In reply to: Joshua D. Drake (#39)

#45

Zeugswetter Andreas SB SD

simon@2ndQuadrant.com

about 19 years ago

In reply to: John Bartlett (#43)

#46

ZeugswetterA@spardat.at

about 19 years ago

In reply to: Simon Riggs (#45)

#47

John Zubac

jzubac@internetsecure.com

about 19 years ago

In reply to: Jim Nasby (#44)

#48

simon@2ndQuadrant.com

about 19 years ago

In reply to: Zeugswetter Andreas SB SD (#46)

#49

jesus@omniti.com

about 19 years ago

In reply to: John Zubac (#47)

#50

jd@commandprompt.com

about 19 years ago

In reply to: Theo Schlossnagle (#49)

#51

rick@alpinenetworking.com

about 19 years ago

In reply to: Joshua D. Drake (#1)

#52

Tom Lane

tgl@sss.pgh.pa.us

about 19 years ago

In reply to: Rick Gigger (#51)

#53

Henry B. Hotz

hotz@jpl.nasa.gov

about 19 years ago

In reply to: Joshua D. Drake (#1)

#54

Bruce Momjian

bruce@momjian.us

about 19 years ago

In reply to: Rick Gigger (#51)

#55

Bruce Momjian

bruce@momjian.us

about 19 years ago

In reply to: Rick Gigger (#51)

#56

Stephen Frost

sfrost@snowman.net

about 19 years ago

In reply to: Henry B. Hotz (#53)

#57

Henry B. Hotz

hotz@jpl.nasa.gov

about 19 years ago

In reply to: Stephen Frost (#56)

#58

Richard Troy

rtroy@ScienceTools.com

about 19 years ago

In reply to: John Bartlett (#42)

#59

Magnus Hagander

magnus@hagander.net

about 19 years ago

In reply to: Henry B. Hotz (#53)

#60

Henry B. Hotz

hotz@jpl.nasa.gov

about 19 years ago

In reply to: Magnus Hagander (#59)

#61

Magnus Hagander

magnus@hagander.net

about 19 years ago

In reply to: Henry B. Hotz (#60)

#62

Chander Ganesan

chander@otg-nc.com

about 19 years ago

In reply to: Joshua D. Drake (#39)

#63

Chander Ganesan

chander@otg-nc.com

about 19 years ago

In reply to: Iannsp (#16)

#64

Chander Ganesan

chander@otg-nc.com

about 19 years ago

In reply to: Chander Ganesan (#63)

#65

Andrew Hammond

andrew.george.hammond@gmail.com

about 19 years ago

In reply to: Tom Lane (#52)

#66

rick@alpinenetworking.com

about 19 years ago

In reply to: Tom Lane (#52)

#67

rick@alpinenetworking.com

about 19 years ago

In reply to: Andrew Hammond (#65)

#68

rick@alpinenetworking.com

about 19 years ago

In reply to: Bruce Momjian (#55)

#69

Jim Nasby

Jim.Nasby@BlueTreble.com

about 19 years ago

In reply to: Andrew Hammond (#65)

#70

Andrew Hammond

andrew.george.hammond@gmail.com

about 19 years ago

In reply to: Jim Nasby (#69)

#71