Обсуждение: `pg_restore --if-exists` clarification
The following documentation comment has been logged on the website: Page: https://www.postgresql.org/docs/16/app-pgrestore.html Description: Good Morning, In the `pg_restore` docs (https://www.postgresql.org/docs/current/app-pgrestore.html), `--if-exists` states that it is to > Use conditional commands (i.e., add an IF EXISTS clause) > to drop database objects. This option is not valid unless > --clean is also specified.", but not being a SQL expert, > I'm having a hard time deciphering this. Suggestion: Would you consider adding the sentence (or something similar)? > "--clean makes pg_restore drop all objects first, > and --if-exists prevents that non-existent objects > cause a failure." Source: https://stackoverflow.com/a/75136163/1498178 This is succinct, and tells exactly what one needs to know. Thank you and have a great day! Appreciatively, Attila
On Thu, Sep 28, 2023, 05:52 PG Doc comments form <noreply@postgresql.org> wrote:
The following documentation comment has been logged on the website:
Page: https://www.postgresql.org/docs/16/app-pgrestore.html
Description:
Good Morning,
In the `pg_restore` docs
(https://www.postgresql.org/docs/current/app-pgrestore.html), `--if-exists`
states that it is to
> Use conditional commands (i.e., add an IF EXISTS clause)
> to drop database objects. This option is not valid unless
> --clean is also specified.", but not being a SQL expert,
> I'm having a hard time deciphering this.
Suggestion: Would you consider adding the sentence (or something similar)?
> "--clean makes pg_restore drop all objects first,
> and --if-exists prevents that non-existent objects
> cause a failure."
Source: https://stackoverflow.com/a/75136163/1498178
This is succinct, and tells exactly what one needs to know. Thank you and
have a great day!
Appreciatively,
Attila
But "failure" is not what happens. If you read the part regarding --clean, you will see that it says:
(Unless
--if-exists
is used, this might generate some harmless error messages, if any objects were not present in the destination database.) [emphasis added]- with extra emphasis on the word "harmless" -- no failure is caused; the restore proceeds just fine.
- If anything were to change in this regard, it might be better to reconsider what we call the message (i.e.what language pg_restore emits in this scenario.) It's true, I suppose, in a literal sense that it's an error in that pg_restore couldn't drop a table when instructed to, where no such table exists. But pragmatically it doesn't matter, so why not reclassify this as a "warning" or a "notice"?
- --Kirk
So `--if-exists` simply suppresses any notice / warning that would occur?
I'm sorry if I come off as obtuse, but I have literally no idea what `--clean` and `--if-exists` do when used together. I took the quote from the Stackoverflow answer on face value, because it sounded plausible and I can't understand `--if-exists` current description yet. Any clarification there would be an improvement in my opinion for people in similar situations (i.e., having little to no SQL knowledge, but still having to do basic DB admin tasks).
Appreciatively,
Attila
On Thu, Sep 28, 2023 at 12:01 PM Kirk Parker <khp@equatoria.us> wrote:
On Thu, Sep 28, 2023, 05:52 PG Doc comments form <noreply@postgresql.org> wrote:The following documentation comment has been logged on the website:
Page: https://www.postgresql.org/docs/16/app-pgrestore.html
Description:
Good Morning,
In the `pg_restore` docs
(https://www.postgresql.org/docs/current/app-pgrestore.html), `--if-exists`
states that it is to
> Use conditional commands (i.e., add an IF EXISTS clause)
> to drop database objects. This option is not valid unless
> --clean is also specified.", but not being a SQL expert,
> I'm having a hard time deciphering this.
Suggestion: Would you consider adding the sentence (or something similar)?
> "--clean makes pg_restore drop all objects first,
> and --if-exists prevents that non-existent objects
> cause a failure."
Source: https://stackoverflow.com/a/75136163/1498178
This is succinct, and tells exactly what one needs to know. Thank you and
have a great day!
Appreciatively,
AttilaBut "failure" is not what happens. If you read the part regarding --clean, you will see that it says:
(Unless
--if-exists
is used, this might generate some harmless error messages, if any objects were not present in the destination database.) [emphasis added]- with extra emphasis on the word "harmless" -- no failure is caused; the restore proceeds just fine.
- If anything were to change in this regard, it might be better to reconsider what we call the message (i.e.what language pg_restore emits in this scenario.) It's true, I suppose, in a literal sense that it's an error in that pg_restore couldn't drop a table when instructed to, where no such table exists. But pragmatically it doesn't matter, so why not reclassify this as a "warning" or a "notice"?
- --Kirk
On Thursday, September 28, 2023, Gulyás Attila <toraritte@gmail.com> wrote:
So `--if-exists` simply suppresses any notice / warning that would occur?
That switch causes the drop DDL produced by pg_restore to include if exists clauses. The server then simply treats the DDL as a no-op if said object being dropped cannot be found.
But "failure" is not what happens. If you read the part regarding --clean, you will see that it says:
(Unless
--if-exists
is used, this might generate some harmless error messages, if any objects were not present in the destination database.) [emphasis added]- with extra emphasis on the word "harmless" -- no failure is caused; the restore proceeds just fine.
- If anything were to change in this regard, it might be better to reconsider what we call the message (i.e.what language pg_restore emits in this scenario.) It's true, I suppose, in a literal sense that it's an error in that pg_restore couldn't drop a table when instructed to, where no such table exists. But pragmatically it doesn't matter, so why not reclassify this as a "warning" or a "notice"?
Because pg_restore is just a client and it is repeating back what the server tells it. And for the server it is an error to drop an object that doesn’t exist.
psql and pg_restore, as clients, can choose to ignore the errors they see, regardless of what kind of error it is, but they don’t take on the added burden of trying to reclassify errors into something else.
David J.
"David G. Johnston" <david.g.johnston@gmail.com> writes: >>> .. But pragmatically it doesn't matter, so why not >>> reclassify this as a "warning" or a "notice"? > Because pg_restore is just a client and it is repeating back what the > server tells it. And for the server it is an error to drop an object that > doesn’t exist. > psql and pg_restore, as clients, can choose to ignore the errors they see, > regardless of what kind of error it is, but they don’t take on the added > burden of trying to reclassify errors into something else. If you actually try this, you'll get output like ... pg_restore: error: could not execute query: ERROR: schema "fkpart6" does not exist Command was: DROP SCHEMA fkpart6; pg_restore: error: could not execute query: ERROR: schema "fkpart5" does not exist Command was: DROP SCHEMA fkpart5; pg_restore: error: could not execute query: ERROR: schema "fkpart4" does not exist Command was: DROP SCHEMA fkpart4; pg_restore: error: could not execute query: ERROR: schema "fkpart3" does not exist Command was: DROP SCHEMA fkpart3; ... pg_restore: warning: errors ignored on restore: 1488 so the only real problem is that there are (typically) enough of these to obscure any errors that might be of greater significance. Still, genuine restore errors would come out after the blizzard of failed DROPs, so I don't think there's a big problem in practice. If we actually wanted to change any behavior here, I think what would be most profitable to discuss is making --if-exists the default. Not sure if we want to go there, but if we'd had the DROP IF EXISTS option all along I bet it would have been done that way. In any case, it's fair to complain about the documentation. How about --clean Before restoring database objects, issue commands to DROP all the objects that will be restored. This option is useful for overwriting an existing database. If any of the objects do not exist in the destination database, ignorable error messages will be reported, unless --if-exists is also specified. --if-exists Use DROP ... IF EXISTS commands to drop objects in --clean mode. This suppresses "does not exist" errors that might otherwise be reported. This option is not valid unless --clean is also specified. regards, tom lane
I would love to see Tom's version in the docs! It is only slightly longer than the current one, but absolutely clear of the purpose of both, and the behaviour when used in tandem.
Appreciatively,
Attila
On Thu, Sep 28, 2023 at 1:03 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
"David G. Johnston" <david.g.johnston@gmail.com> writes:
>>> .. But pragmatically it doesn't matter, so why not
>>> reclassify this as a "warning" or a "notice"?
> Because pg_restore is just a client and it is repeating back what the
> server tells it. And for the server it is an error to drop an object that
> doesn’t exist.
> psql and pg_restore, as clients, can choose to ignore the errors they see,
> regardless of what kind of error it is, but they don’t take on the added
> burden of trying to reclassify errors into something else.
If you actually try this, you'll get output like
...
pg_restore: error: could not execute query: ERROR: schema "fkpart6" does not exist
Command was: DROP SCHEMA fkpart6;
pg_restore: error: could not execute query: ERROR: schema "fkpart5" does not exist
Command was: DROP SCHEMA fkpart5;
pg_restore: error: could not execute query: ERROR: schema "fkpart4" does not exist
Command was: DROP SCHEMA fkpart4;
pg_restore: error: could not execute query: ERROR: schema "fkpart3" does not exist
Command was: DROP SCHEMA fkpart3;
...
pg_restore: warning: errors ignored on restore: 1488
so the only real problem is that there are (typically) enough of these
to obscure any errors that might be of greater significance. Still,
genuine restore errors would come out after the blizzard of failed
DROPs, so I don't think there's a big problem in practice.
If we actually wanted to change any behavior here, I think what would
be most profitable to discuss is making --if-exists the default.
Not sure if we want to go there, but if we'd had the DROP IF EXISTS
option all along I bet it would have been done that way.
In any case, it's fair to complain about the documentation.
How about
--clean
Before restoring database objects, issue commands to DROP all the
objects that will be restored. This option is useful for overwriting
an existing database. If any of the objects do not exist in the
destination database, ignorable error messages will be reported,
unless --if-exists is also specified.
--if-exists
Use DROP ... IF EXISTS commands to drop objects in --clean mode.
This suppresses "does not exist" errors that might otherwise be
reported. This option is not valid unless --clean is also specified.
regards, tom lane
=?UTF-8?Q?Guly=C3=A1s_Attila?= <toraritte@gmail.com> writes: > I would love to see Tom's version in the docs! It is only slightly longer > than the current one, but absolutely clear of the purpose of both, and the > behaviour when used in tandem. Hearing no objections, done at https://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=75af0f401f905b947ea14401e8a51f1bae4ac265 regards, tom lane
Thank you so much!
Appreciatively,
Attila
On Fri, Sep 29, 2023 at 1:15 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
Gulyás Attila <toraritte@gmail.com> writes:
> I would love to see Tom's version in the docs! It is only slightly longer
> than the current one, but absolutely clear of the purpose of both, and the
> behaviour when used in tandem.
Hearing no objections, done at
https://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=75af0f401f905b947ea14401e8a51f1bae4ac265
regards, tom lane