Re: POC PATCH: copy from ... exceptions to: (was Re: VLDB Features)

2014-12-25 22:23 GMT+01:00 Alex Shulgin <ash(at)commandprompt(dot)com>:

> Trent Shipley <trent_shipley(at)qwest(dot)net> writes:
>
> > On Friday 2007-12-14 16:22, Tom Lane wrote:
> >> Neil Conway <neilc(at)samurai(dot)com> writes:
> >> > By modifying COPY: COPY IGNORE ERRORS or some such would instruct COPY
> >> > to drop (and log) rows that contain malformed data. That is, rows with
> >> > too many or too few columns, rows that result in constraint
> violations,
> >> > and rows containing columns where the data type's input function
> raises
> >> > an error. The last case is the only thing that would be a bit tricky
> to
> >> > implement, I think: you could use PG_TRY() around the
> InputFunctionCall,
> >> > but I guess you'd need a subtransaction to ensure that you reset your
> >> > state correctly after catching an error.
> >>
> >> Yeah. It's the subtransaction per row that's daunting --- not only the
> >> cycles spent for that, but the ensuing limitation to 4G rows imported
> >> per COPY.
> >
> > You could extend the COPY FROM syntax with a COMMIT EVERY n clause. This
> > would help with the 4G subtransaction limit. The cost to the ETL
> process is
> > that a simple rollback would not be guaranteed send the process back to
> it's
> > initial state. There are easy ways to deal with the rollback issue
> though.
> >
> > A {NO} RETRY {USING algorithm} clause might be useful. If the NO RETRY
> > option is selected then the COPY FROM can run without subtransactions
> and in
> > excess of the 4G per transaction limit. NO RETRY should be the default
> since
> > it preserves the legacy behavior of COPY FROM.
> >
> > You could have an EXCEPTIONS TO {filename|STDERR} clause. I would not
> give the
> > option of sending exceptions to a table since they are presumably
> malformed,
> > otherwise they would not be exceptions. (Users should re-process
> exception
> > files if they want an if good then table a else exception to table b ...)
> >
> > EXCEPTIONS TO and NO RETRY would be mutually exclusive.
> >
> >
> >> If we could somehow only do a subtransaction per failure, things would
> >> be much better, but I don't see how.
>
> Hello,
>
> Attached is a proof of concept patch for this TODO item. There is no
> docs yet, I just wanted to know if approach is sane.
>
> The added syntax is like the following:
>
> COPY [table] FROM [file/program/stdin] EXCEPTIONS TO [file or stdout]
>
> The way it's done it is abusing Copy Both mode and from my limited
> testing, that seems to just work. The error trapping itself is done
> using PG_TRY/PG_CATCH and can only catch formatting or before-insert
> trigger errors, no attempt is made to recover from a failed unique
> constraint, etc.
>
> Example in action:
>
> postgres=# \d test_copy2
> Table "public.test_copy2"
> Column | Type | Modifiers
> --------+---------+-----------
> id | integer |
> val | integer |
>
> postgres=# copy test_copy2 from program 'seq 3' exceptions to stdout;
> 1
> NOTICE: missing data for column "val"
> CONTEXT: COPY test_copy2, line 1: "1"
> 2
> NOTICE: missing data for column "val"
> CONTEXT: COPY test_copy2, line 2: "2"
> 3
> NOTICE: missing data for column "val"
> CONTEXT: COPY test_copy2, line 3: "3"
> NOTICE: total exceptions ignored: 3
>
> postgres=# \d test_copy1
> Table "public.test_copy1"
> Column | Type | Modifiers
> --------+---------+-----------
> id | integer | not null
>
> postgres=# set client_min_messages to warning;
> SET
> postgres=# copy test_copy1 from program 'ls /proc' exceptions to stdout;
> ...
> vmstat
> zoneinfo
> postgres=#
>
> Limited performance testing shows no significant difference between
> error-catching and plain code path. For example, timing
>
> copy test_copy1 from program 'seq 1000000' [exceptions to stdout]
>
> shows similar numbers with or without the added "exceptions to" clause.
>
> Now that I'm sending this I wonder if the original comment about the
> need for subtransaction around every loaded line still holds. Any
> example of what would be not properly rolled back by just PG_TRY?
>

this method is unsafe .. exception handlers doesn't free memory usually -
there is risk of memory leaks, source leaks

you can enforce same performance with block subtransactions - when you use
subtransaction for 1000 rows, then impact of subtransactions is minimal

when block fails, then you can use row level subtransaction - it works well
when you expect almost correct data.

Regards

Pavel

>
> Happy hacking!
> --
> Alex
>
>
>
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