Thread: Re: [HACKERS] TRUNCATE TABLE with IDENTITY

Zoltan Boszormenyi írta:
> Decibel! írta:
>> On Apr 3, 2008, at 12:52 AM, Zoltan Boszormenyi wrote:
>>> Where is the info in the sequence to provide restarting with
>>> the _original_ start value?
>>
>> There isn't any. If you want the sequence to start at some magic
>> value, adjust the minimum value.
>
> There's the START WITH option for IDENTITY columns and this below
> is paragraph 8 under General rules of 14.10 <truncate table statement>
> in 6WD2_02_Foundation_2007-12.pdf (page 902):
>
> 8) If RESTART IDENTITY is specified and the table descriptor of T
> includes a column descriptor IDCD of
>   an identity column, then:
>   a) Let CN be the column name included in IDCD and let SV be the
> start value included in IDCD.
>   b) The following <alter table statement> is effectively executed
> without further Access Rule checking:
>       ALTER TABLE TN ALTER COLUMN CN RESTART WITH SV
>
> This says that the original start value is used, not the minimum value.
> IDENTITY has the same options as CREATE SEQUENCE. In fact the
> "identity column specification" links to "11.63 <sequence generator
> definition>"
> when it comes to IDENTITY sequence options. And surprise, surprise,
> "11.64 <alter sequence generator statement>" now defines
> ALTER SEQUENCE sn RESTART [WITH newvalue]
> where omitting the "WITH newval" part also uses the original start value.
>
> Best regards,
> Zoltán Böszörményi

Attached patch implements the extension found in the current SQL200n draft,
implementing stored start value and supporting ALTER SEQUENCE seq RESTART;
Some error check are also added to prohibit CREATE SEQUENCE ... RESTART ...
and ALTER SEQUENCE ... START ...

Best regards,
Zoltán Böszörményi

--
----------------------------------
Zoltán Böszörményi
Cybertec Schönig & Schönig GmbH
http://www.postgresql.at/

diff -dcrpN pgsql.orig/src/backend/commands/sequence.c pgsql/src/backend/commands/sequence.c
*** pgsql.orig/src/backend/commands/sequence.c    2008-01-01 20:45:49.000000000 +0100
--- pgsql/src/backend/commands/sequence.c    2008-04-08 10:51:27.000000000 +0200
*************** static Relation open_share_lock(SeqTable
*** 88,94 ****
  static void init_sequence(Oid relid, SeqTable *p_elm, Relation *p_rel);
  static Form_pg_sequence read_info(SeqTable elm, Relation rel, Buffer *buf);
  static void init_params(List *options, bool isInit,
!             Form_pg_sequence new, List **owned_by);
  static void do_setval(Oid relid, int64 next, bool iscalled);
  static void process_owned_by(Relation seqrel, List *owned_by);

--- 88,94 ----
  static void init_sequence(Oid relid, SeqTable *p_elm, Relation *p_rel);
  static Form_pg_sequence read_info(SeqTable elm, Relation rel, Buffer *buf);
  static void init_params(List *options, bool isInit,
!             Form_pg_sequence new, Form_pg_sequence old, List **owned_by);
  static void do_setval(Oid relid, int64 next, bool iscalled);
  static void process_owned_by(Relation seqrel, List *owned_by);

*************** DefineSequence(CreateSeqStmt *seq)
*** 116,122 ****
      NameData    name;

      /* Check and set all option values */
!     init_params(seq->options, true, &new, &owned_by);

      /*
       * Create relation (and fill *null & *value)
--- 116,122 ----
      NameData    name;

      /* Check and set all option values */
!     init_params(seq->options, true, &new, NULL, &owned_by);

      /*
       * Create relation (and fill *null & *value)
*************** DefineSequence(CreateSeqStmt *seq)
*** 143,148 ****
--- 143,153 ----
                  namestrcpy(&name, seq->sequence->relname);
                  value[i - 1] = NameGetDatum(&name);
                  break;
+             case SEQ_COL_STARTVAL:
+                 coldef->typename = makeTypeNameFromOid(INT8OID, -1);
+                 coldef->colname = "start_value";
+                 value[i - 1] = Int64GetDatumFast(new.start_value);
+                 break;
              case SEQ_COL_LASTVAL:
                  coldef->typename = makeTypeNameFromOid(INT8OID, -1);
                  coldef->colname = "last_value";
*************** AlterSequence(AlterSeqStmt *stmt)
*** 336,342 ****
      memcpy(&new, seq, sizeof(FormData_pg_sequence));

      /* Check and set new values */
!     init_params(stmt->options, false, &new, &owned_by);

      /* Clear local cache so that we don't think we have cached numbers */
      /* Note that we do not change the currval() state */
--- 341,347 ----
      memcpy(&new, seq, sizeof(FormData_pg_sequence));

      /* Check and set new values */
!     init_params(stmt->options, false, &new, seq, &owned_by);

      /* Clear local cache so that we don't think we have cached numbers */
      /* Note that we do not change the currval() state */
*************** read_info(SeqTable elm, Relation rel, Bu
*** 967,973 ****
   */
  static void
  init_params(List *options, bool isInit,
!             Form_pg_sequence new, List **owned_by)
  {
      DefElem    *last_value = NULL;
      DefElem    *increment_by = NULL;
--- 972,978 ----
   */
  static void
  init_params(List *options, bool isInit,
!             Form_pg_sequence new, Form_pg_sequence old, List **owned_by)
  {
      DefElem    *last_value = NULL;
      DefElem    *increment_by = NULL;
*************** init_params(List *options, bool isInit,
*** 995,1003 ****
          /*
           * start is for a new sequence restart is for alter
           */
!         else if (strcmp(defel->defname, "start") == 0 ||
!                  strcmp(defel->defname, "restart") == 0)
          {
              if (last_value)
                  ereport(ERROR,
                          (errcode(ERRCODE_SYNTAX_ERROR),
--- 1000,1023 ----
          /*
           * start is for a new sequence restart is for alter
           */
!         else if (strcmp(defel->defname, "start") == 0)
          {
+             if (!isInit)
+                 ereport(ERROR,
+                         (errcode(ERRCODE_SYNTAX_ERROR),
+                          errmsg("use ALTER SEQUENCE ... RESTART ...")));
+             if (last_value)
+                 ereport(ERROR,
+                         (errcode(ERRCODE_SYNTAX_ERROR),
+                          errmsg("conflicting or redundant options")));
+             last_value = defel;
+         }
+         else if (strcmp(defel->defname, "restart") == 0)
+         {
+             if (isInit)
+                 ereport(ERROR,
+                         (errcode(ERRCODE_SYNTAX_ERROR),
+                          errmsg("use CREATE SEQUENCE ... START ...")));
              if (last_value)
                  ereport(ERROR,
                          (errcode(ERRCODE_SYNTAX_ERROR),
*************** init_params(List *options, bool isInit,
*** 1109,1124 ****
      /* START WITH */
      if (last_value != NULL)
      {
!         new->last_value = defGetInt64(last_value);
          new->is_called = false;
          new->log_cnt = 1;
      }
      else if (isInit)
      {
          if (new->increment_by > 0)
!             new->last_value = new->min_value;    /* ascending seq */
          else
!             new->last_value = new->max_value;    /* descending seq */
          new->is_called = false;
          new->log_cnt = 1;
      }
--- 1129,1149 ----
      /* START WITH */
      if (last_value != NULL)
      {
!         if (last_value->arg != NULL)
!             new->last_value = defGetInt64(last_value);
!         else
!             new->last_value = old->start_value;
!         if (isInit)
!             new->start_value = new->last_value;
          new->is_called = false;
          new->log_cnt = 1;
      }
      else if (isInit)
      {
          if (new->increment_by > 0)
!             new->start_value = new->last_value = new->min_value;    /* ascending seq */
          else
!             new->start_value = new->last_value = new->max_value;    /* descending seq */
          new->is_called = false;
          new->log_cnt = 1;
      }
diff -dcrpN pgsql.orig/src/backend/parser/gram.y pgsql/src/backend/parser/gram.y
*** pgsql.orig/src/backend/parser/gram.y    2008-03-30 13:29:47.000000000 +0200
--- pgsql/src/backend/parser/gram.y    2008-04-08 10:32:25.000000000 +0200
*************** OptSeqElem: CACHE NumericOnly
*** 2486,2491 ****
--- 2486,2495 ----
                  {
                      $$ = makeDefElem("start", (Node *)$3);
                  }
+             | RESTART
+                 {
+                     $$ = makeDefElem("restart", NULL);
+                 }
              | RESTART opt_with NumericOnly
                  {
                      $$ = makeDefElem("restart", (Node *)$3);
diff -dcrpN pgsql.orig/src/include/commands/sequence.h pgsql/src/include/commands/sequence.h
*** pgsql.orig/src/include/commands/sequence.h    2008-03-30 13:29:50.000000000 +0200
--- pgsql/src/include/commands/sequence.h    2008-04-08 10:22:28.000000000 +0200
*************** typedef struct FormData_pg_sequence
*** 29,34 ****
--- 29,35 ----
  {
      NameData    sequence_name;
  #ifndef INT64_IS_BUSTED
+     int64        start_value;
      int64        last_value;
      int64        increment_by;
      int64        max_value;
*************** typedef struct FormData_pg_sequence
*** 36,53 ****
      int64        cache_value;
      int64        log_cnt;
  #else
!     int32        last_value;
      int32        pad1;
!     int32        increment_by;
      int32        pad2;
!     int32        max_value;
      int32        pad3;
!     int32        min_value;
      int32        pad4;
!     int32        cache_value;
      int32        pad5;
!     int32        log_cnt;
      int32        pad6;
  #endif
      bool        is_cycled;
      bool        is_called;
--- 37,56 ----
      int64        cache_value;
      int64        log_cnt;
  #else
!     int32        start_value;
      int32        pad1;
!     int32        last_value;
      int32        pad2;
!     int32        increment_by;
      int32        pad3;
!     int32        max_value;
      int32        pad4;
!     int32        min_value;
      int32        pad5;
!     int32        cache_value;
      int32        pad6;
+     int32        log_cnt;
+     int32        pad7;
  #endif
      bool        is_cycled;
      bool        is_called;
*************** typedef FormData_pg_sequence *Form_pg_se
*** 60,73 ****
   */

  #define SEQ_COL_NAME            1
! #define SEQ_COL_LASTVAL            2
! #define SEQ_COL_INCBY            3
! #define SEQ_COL_MAXVALUE        4
! #define SEQ_COL_MINVALUE        5
! #define SEQ_COL_CACHE            6
! #define SEQ_COL_LOG                7
! #define SEQ_COL_CYCLE            8
! #define SEQ_COL_CALLED            9

  #define SEQ_COL_FIRSTCOL        SEQ_COL_NAME
  #define SEQ_COL_LASTCOL            SEQ_COL_CALLED
--- 63,77 ----
   */

  #define SEQ_COL_NAME            1
! #define SEQ_COL_STARTVAL        2
! #define SEQ_COL_LASTVAL            3
! #define SEQ_COL_INCBY            4
! #define SEQ_COL_MAXVALUE        5
! #define SEQ_COL_MINVALUE        6
! #define SEQ_COL_CACHE            7
! #define SEQ_COL_LOG            8
! #define SEQ_COL_CYCLE            9
! #define SEQ_COL_CALLED            10

  #define SEQ_COL_FIRSTCOL        SEQ_COL_NAME
  #define SEQ_COL_LASTCOL            SEQ_COL_CALLED
diff -dcrpN pgsql.orig/src/test/regress/expected/sequence.out pgsql/src/test/regress/expected/sequence.out
*** pgsql.orig/src/test/regress/expected/sequence.out    2006-08-21 02:57:26.000000000 +0200
--- pgsql/src/test/regress/expected/sequence.out    2008-04-08 10:58:18.000000000 +0200
*************** DROP SEQUENCE sequence_test;
*** 99,107 ****
  CREATE SEQUENCE foo_seq;
  ALTER TABLE foo_seq RENAME TO foo_seq_new;
  SELECT * FROM foo_seq_new;
!  sequence_name | last_value | increment_by |      max_value      | min_value | cache_value | log_cnt | is_cycled |
is_called 
!
---------------+------------+--------------+---------------------+-----------+-------------+---------+-----------+-----------
!  foo_seq       |          1 |            1 | 9223372036854775807 |         1 |           1 |       1 | f         | f
  (1 row)

  DROP SEQUENCE foo_seq_new;
--- 99,107 ----
  CREATE SEQUENCE foo_seq;
  ALTER TABLE foo_seq RENAME TO foo_seq_new;
  SELECT * FROM foo_seq_new;
!  sequence_name | start_value | last_value | increment_by |      max_value      | min_value | cache_value | log_cnt |
is_cycled| is_called  
!
---------------+-------------+------------+--------------+---------------------+-----------+-------------+---------+-----------+-----------
!  foo_seq       |           1 |          1 |            1 | 9223372036854775807 |         1 |           1 |       1 |
f        | f 
  (1 row)

  DROP SEQUENCE foo_seq_new;
*************** SELECT nextval('sequence_test2');
*** 175,180 ****
--- 175,212 ----
         5
  (1 row)

+ ALTER SEQUENCE sequence_test2 RESTART
+      INCREMENT BY 4 MAXVALUE 36 MINVALUE 5 CYCLE;
+ SELECT nextval('sequence_test2');
+  nextval
+ ---------
+       32
+ (1 row)
+
+ SELECT nextval('sequence_test2');
+  nextval
+ ---------
+       36
+ (1 row)
+
+ SELECT nextval('sequence_test2');
+  nextval
+ ---------
+        5
+ (1 row)
+
+ SELECT nextval('sequence_test2');
+  nextval
+ ---------
+        9
+ (1 row)
+
+ SELECT nextval('sequence_test2');
+  nextval
+ ---------
+       13
+ (1 row)
+
  -- Test comments
  COMMENT ON SEQUENCE asdf IS 'won''t work';
  ERROR:  relation "asdf" does not exist
diff -dcrpN pgsql.orig/src/test/regress/sql/sequence.sql pgsql/src/test/regress/sql/sequence.sql
*** pgsql.orig/src/test/regress/sql/sequence.sql    2005-10-03 01:50:16.000000000 +0200
--- pgsql/src/test/regress/sql/sequence.sql    2008-04-08 10:56:55.000000000 +0200
*************** SELECT nextval('sequence_test2');
*** 74,79 ****
--- 74,88 ----
  SELECT nextval('sequence_test2');
  SELECT nextval('sequence_test2');

+ ALTER SEQUENCE sequence_test2 RESTART
+      INCREMENT BY 4 MAXVALUE 36 MINVALUE 5 CYCLE;
+
+ SELECT nextval('sequence_test2');
+ SELECT nextval('sequence_test2');
+ SELECT nextval('sequence_test2');
+ SELECT nextval('sequence_test2');
+ SELECT nextval('sequence_test2');
+
  -- Test comments
  COMMENT ON SEQUENCE asdf IS 'won''t work';
  COMMENT ON SEQUENCE sequence_test2 IS 'will work';

Zoltan Boszormenyi írta:
> Zoltan Boszormenyi írta:
>> Decibel! írta:
>>> On Apr 3, 2008, at 12:52 AM, Zoltan Boszormenyi wrote:
>>>> Where is the info in the sequence to provide restarting with
>>>> the _original_ start value?
>>>
>>> There isn't any. If you want the sequence to start at some magic
>>> value, adjust the minimum value.
>>
>> There's the START WITH option for IDENTITY columns and this below
>> is paragraph 8 under General rules of 14.10 <truncate table statement>
>> in 6WD2_02_Foundation_2007-12.pdf (page 902):
>>
>> 8) If RESTART IDENTITY is specified and the table descriptor of T
>> includes a column descriptor IDCD of
>>   an identity column, then:
>>   a) Let CN be the column name included in IDCD and let SV be the
>> start value included in IDCD.
>>   b) The following <alter table statement> is effectively executed
>> without further Access Rule checking:
>>       ALTER TABLE TN ALTER COLUMN CN RESTART WITH SV
>>
>> This says that the original start value is used, not the minimum value.
>> IDENTITY has the same options as CREATE SEQUENCE. In fact the
>> "identity column specification" links to "11.63 <sequence generator
>> definition>"
>> when it comes to IDENTITY sequence options. And surprise, surprise,
>> "11.64 <alter sequence generator statement>" now defines
>> ALTER SEQUENCE sn RESTART [WITH newvalue]
>> where omitting the "WITH newval" part also uses the original start
>> value.
>>
>> Best regards,
>> Zoltán Böszörményi
>
> Attached patch implements the extension found in the current SQL200n
> draft,
> implementing stored start value and supporting ALTER SEQUENCE seq
> RESTART;
> Some error check are also added to prohibit CREATE SEQUENCE ...
> RESTART ...
> and ALTER SEQUENCE ... START ...
>
> Best regards,
> Zoltán Böszörményi

Updated patch implements TRUNCATE ... RESTART IDENTITY
which restarts all owned sequences for the truncated table(s).
Regression tests updated, documentation added. pg_dump was
also extended to output original[1] START value for creating SEQUENCEs.

[1] For 8.3 and below I could only guesstimate it as MINVALUE for ascending
      and MAXVALUE for descending sequences.

Best regards,
Zoltán Böszörményi

--
----------------------------------
Zoltán Böszörményi
Cybertec Schönig & Schönig GmbH
http://www.postgresql.at/

Zoltan Boszormenyi <zb@cybertec.at> writes:
>> Attached patch implements the extension found in the current SQL200n draft,
>> implementing stored start value and supporting ALTER SEQUENCE seq RESTART;

> Updated patch implements TRUNCATE ... RESTART IDENTITY
> which restarts all owned sequences for the truncated table(s).

Applied with corrections.  Most notably, since ALTER SEQUENCE RESTART
is nontransactional like most other ALTER SEQUENCE operations, I
rearranged things to try to ensure that foreseeable failures like
deadlock and lack of permissions would be detected before TRUNCATE
starts to issue any RESTART commands.

One interesting point here is that the patch as submitted allowed
ALTER SEQUENCE MINVALUE/MAXVALUE to be used to set a sequence range
that the original START value was outside of.  This would result in
a failure at ALTER SEQUENCE RESTART.  Since, as stated above, we
really don't want that happening during TRUNCATE, I adjusted the
patch to make such an ALTER SEQUENCE fail.  This is at least potentially
an incompatible change: command sequences that used to be legal could
now fail.  I doubt it's very likely to bite anyone in practice, though.

            regards, tom lane

On Fri, 2008-05-16 at 19:41 -0400, Tom Lane wrote:
> Applied with corrections.  Most notably, since ALTER SEQUENCE RESTART
> is nontransactional like most other ALTER SEQUENCE operations, I
> rearranged things to try to ensure that foreseeable failures like
> deadlock and lack of permissions would be detected before TRUNCATE
> starts to issue any RESTART commands.

Ugh. The fact that the RESTART IDENTITY part of TRUNCATE is
non-transactional is a pretty unsightly wort. I would also quarrel with
your addition to the docs that suggests this is only an issue "in
practice" if TRUNCATE RESTART IDENTITY is used in a transaction block:
unpredictable failures (such as OOM or query cancellation) can certainly
occur in practice, and would be very disruptive (e.g. if the sequence
values are stored into a column with a UNIQUE constraint, it would break
all inserting transactions until the DBA intervenes).

I wonder if it would be possible to make the sequence operations
performed by TRUNCATE transactional: while the TRUNCATE remains
uncommitted, it should be okay to block concurrent access to the
sequence.

-Neil

Neil Conway <neilc@samurai.com> writes:
> Ugh. The fact that the RESTART IDENTITY part of TRUNCATE is
> non-transactional is a pretty unsightly wort.

Actually, I agree.  Shall we just revert that feature?  The ALTER
SEQUENCE part of this patch is clean and useful, but I'm less than
enamored of the TRUNCATE part.

            regards, tom lane

On Fri, 2008-05-16 at 21:50 -0400, Tom Lane wrote:
> Neil Conway <neilc@samurai.com> writes:
> > Ugh. The fact that the RESTART IDENTITY part of TRUNCATE is
> > non-transactional is a pretty unsightly wort.
>
> Actually, I agree.  Shall we just revert that feature?  The ALTER
> SEQUENCE part of this patch is clean and useful, but I'm less than
> enamored of the TRUNCATE part.

Perhaps, but we should also take into account that TRUNCATE is not and
never will be MVCC compliant, so its not something you'd expect to run
except as a maintenance action. If we do keep it, I would suggest that
we add a WARNING so that the effects are clearly recorded.

--
 Simon Riggs           www.2ndQuadrant.com
 PostgreSQL Training, Services and Support

Simon Riggs <simon@2ndquadrant.com> writes:
> On Fri, 2008-05-16 at 21:50 -0400, Tom Lane wrote:
>> Actually, I agree.  Shall we just revert that feature?

> Perhaps, but we should also take into account that TRUNCATE is not and
> never will be MVCC compliant, so its not something you'd expect to run
> except as a maintenance action.

Good point.  I had a couple of further thoughts this morning:

1. The case Neil is worried about is something like

    BEGIN;
    TRUNCATE TABLE foo RESTART IDENTITY;
    COPY foo FROM ...;
    COMMIT;

If the COPY fails partway through, the old table contents are restored,
but the sequences are not.  However removing RESTART IDENTITY will not
remove the hazard, because there is no difference between this and

    BEGIN;
    TRUNCATE TABLE foo;
    SELECT setval('foo_id', 1);
    COPY foo FROM ...;
    COMMIT;

other than the latter adding a little extra chance for pilot error in
resetting the wrong sequence.  So if we revert the patch we haven't
accomplished much except to take away an opportunity to point out the
risk.  I vote for leaving the patch in and rewriting the <warning>
to point out this risk.

2. I had first dismissed Neil's idea of transactional sequence updates
as impossible, but on second look it could be done.  Suppose RESTART
IDENTITY does this for each sequence;

    * obtain AccessExclusiveLock;
    * assign a new relfilenode;
    * insert a sequence row with all parameters copied except
      last_value copies start_value;
    * hold AccessExclusiveLock till commit.

IOW just like truncate-and-reload, but for a sequence.  Within the
current backend, subsequent operations see the new sequence values.
If the transaction rolls back, the old sequence relfilenode is still
there and untouched.

It's slightly annoying to need to lock out other backends' nextval
operations, but for the use-case of TRUNCATE this doesn't seem
like it's really much of a problem.

I'm not sure if it'd be worth exposing this behavior as a separate
user-visible command (CREATE OR REPLACE SEQUENCE, maybe?), but it seems
worth doing to make TRUNCATE-and-reload less of a foot gun.

So what I think we should do is leave the patch there, revise the
warning per Neil's complaint, and add a TODO item to reimplement RESTART
IDENTITY transactionally.

            regards, tom lane

On Sat, 2008-05-17 at 12:04 -0400, Tom Lane wrote:

> So what I think we should do is leave the patch there, revise the
> warning per Neil's complaint, and add a TODO item to reimplement
> RESTART IDENTITY transactionally.

Sounds good.

--
 Simon Riggs           www.2ndQuadrant.com
 PostgreSQL Training, Services and Support

Tom Lane wrote:

> 2. I had first dismissed Neil's idea of transactional sequence updates
> as impossible, but on second look it could be done.  Suppose RESTART
> IDENTITY does this for each sequence;
>
>     * obtain AccessExclusiveLock;
>     * assign a new relfilenode;
>     * insert a sequence row with all parameters copied except
>       last_value copies start_value;
>     * hold AccessExclusiveLock till commit.

Hmm, this kills the idea of moving sequence data to a single
non-transactional catalog :-(

> So what I think we should do is leave the patch there, revise the
> warning per Neil's complaint, and add a TODO item to reimplement RESTART
> IDENTITY transactionally.

I think the TODO item did not make it, but the docs do seem updated.

--
Alvaro Herrera                                http://www.CommandPrompt.com/
PostgreSQL Replication, Consulting, Custom Development, 24x7 support

Alvaro Herrera <alvherre@commandprompt.com> writes:
> Tom Lane wrote:
>> 2. I had first dismissed Neil's idea of transactional sequence updates
>> as impossible, but on second look it could be done.  Suppose RESTART
>> IDENTITY does this for each sequence;
>>
>> * obtain AccessExclusiveLock;
>> * assign a new relfilenode;
>> * insert a sequence row with all parameters copied except
>> last_value copies start_value;
>> * hold AccessExclusiveLock till commit.

> Hmm, this kills the idea of moving sequence data to a single
> non-transactional catalog :-(

Well, there are a number of holes in our ideas of how to do that anyway.
But offhand I don't see why we couldn't distinguish regular heap_update
from update_in_place on single rows within a catalog.

            regards, tom lane

Alvaro Herrera wrote:
> Tom Lane wrote:
>
> > 2. I had first dismissed Neil's idea of transactional sequence updates
> > as impossible, but on second look it could be done.  Suppose RESTART
> > IDENTITY does this for each sequence;
> >
> >     * obtain AccessExclusiveLock;
> >     * assign a new relfilenode;
> >     * insert a sequence row with all parameters copied except
> >       last_value copies start_value;
> >     * hold AccessExclusiveLock till commit.
>
> Hmm, this kills the idea of moving sequence data to a single
> non-transactional catalog :-(
>
> > So what I think we should do is leave the patch there, revise the
> > warning per Neil's complaint, and add a TODO item to reimplement RESTART
> > IDENTITY transactionally.
>
> I think the TODO item did not make it, but the docs do seem updated.

Done:

* Fix TRUNCATE ... RESTART IDENTITY so its affect on sequences is rolled
  back on transaction abort

--
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + If your life is a hard drive, Christ can be your backup. +