Should CSV parsing be stricter about mid-field quotes?

čt 11. 5. 2023 v 16:04 odesílatel Joel Jacobson <joel@compiler.org> napsal:

Hi hackers,

I've come across an unexpected behavior in our CSV parser that I'd like to
bring up for discussion.

% cat example.csv
id,rating,review
1,5,"Great product, will buy again."
2,3,"I bought this for my 6" laptop but it didn't fit my 8" tablet"

% psql
CREATE TABLE reviews (id int, rating int, review text);
\COPY reviews FROM example.csv WITH CSV HEADER;
SELECT * FROM reviews;

This gives:

id | rating | review
----+--------+-------------------------------------------------------------
1 | 5 | Great product, will buy again.
2 | 3 | I bought this for my 6 laptop but it didn't fit my 8 tablet
(2 rows)

The parser currently accepts quoting within an unquoted field. This can
lead to
data misinterpretation when the quote is part of the field data (e.g.,
for inches, like in the example).

Our CSV output rules quote an entire field or not at all. But the import of
fields with mid-field quotes might lead to surprising and undetected
outcomes.

I think we should throw a parsing error for unescaped mid-field quotes,
and add a COPY option like ALLOW_MIDFIELD_QUOTES for cases where mid-field
quotes are necessary. The error message could suggest this option when it
encounters an unescaped mid-field quote.

I think the convenience of not having to use an extra option doesn't
outweigh
the risk of undetected data integrity issues.

Thoughts?

+1

Pavel

On Thu, 11 May 2023 at 10:04, Joel Jacobson <joel@compiler.org> wrote:

The parser currently accepts quoting within an unquoted field. This can lead to
data misinterpretation when the quote is part of the field data (e.g.,
for inches, like in the example).

I think you're thinking about it differently than the parser. I think
the parser is treating this the way, say, the shell treats quotes.
That is, it sees a quoted "I bought this for my 6" followed by an
unquoted "a laptop but it didn't fit my 8" followed by a quoted "
tablet".

So for example, in that world you might only quote commas and newlines
so you might print something like

1,2,I bought this for my "6"" laptop
" but it "didn't" fit my "8""" laptop

The actual CSV spec https://datatracker.ietf.org/doc/html/rfc4180 only
allows fully quoted or fully unquoted fields and there can only be
escaped double-doublequote characters in quoted fields and no
doublequote characters in unquoted fields.

But it also says

Due to lack of a single specification, there are considerable
differences among implementations. Implementors should "be
conservative in what you do, be liberal in what you accept from
others" (RFC 793 [8]) when processing CSV files. An attempt at a
common definition can be found in Section 2.

So the real question is are there tools out there that generate
entries like this and what are their intentions?

I think we should throw a parsing error for unescaped mid-field quotes,
and add a COPY option like ALLOW_MIDFIELD_QUOTES for cases where mid-field
quotes are necessary. The error message could suggest this option when it
encounters an unescaped mid-field quote.

I think the convenience of not having to use an extra option doesn't outweigh
the risk of undetected data integrity issues.

It's also a pretty annoying experience to get a message saying "error,
turn this option on to not get an error". I get what you're saying
too, which is more of a risk depends on whether turning off the error
is really the right thing most of the time or is just causing data to
be read incorrectly.

--
greg

On 2023-05-11 Th 10:03, Joel Jacobson wrote:

Hi hackers,

I've come across an unexpected behavior in our CSV parser that I'd like to
bring up for discussion.

% cat example.csv
id,rating,review
1,5,"Great product, will buy again."
2,3,"I bought this for my 6" laptop but it didn't fit my 8" tablet"

% psql
CREATE TABLE reviews (id int, rating int, review text);
\COPY reviews FROM example.csv WITH CSV HEADER;
SELECT * FROM reviews;

This gives:

id | rating |                           review
----+--------+-------------------------------------------------------------
  1 |      5 | Great product, will buy again.
  2 |      3 | I bought this for my 6 laptop but it didn't fit my 8 tablet
(2 rows)

Maybe this is unexpected by you, but it's not by me. What other sane
interpretation of that data could there be? And what CSV producer
outputs such horrible content? As you've noted, ours certainly does not.
Our rules are clear: quotes within quotes must be escaped (default
escape is by doubling the quote char). Allowing partial fields to be
quoted was a deliberate decision when CSV parsing was implemented,
because examples have been seen in the wild.

So I don't think our behaviour is broken or needs fixing. As mentioned
by Greg, this is an example of the adage about being liberal in what you
accept.

cheers

andrew

--
Andrew Dunstan
EDB:https://www.enterprisedb.com

On Fri, May 12, 2023, at 21:57, Andrew Dunstan wrote:

Maybe this is unexpected by you, but it's not by me. What other sane interpretation of that data could there be? And what CSV producer outputs such horrible content? As you've noted, ours certainly does not. Our rules are clear: quotes within quotes must be escaped (default escape is by doubling the quote char). Allowing partial fields to be quoted was a deliberate decision when CSV parsing was implemented, because examples have been seen in the wild.

So I don't think our behaviour is broken or needs fixing. As mentioned by Greg, this is an example of the adage about being liberal in what you accept.

I understand your position, and your points are indeed in line with the
traditional "Robustness Principle" (aka "Postel's Law") [1]https://datatracker.ietf.org/doc/html/rfc761#section-2.10 from 1980, which
suggests "be conservative in what you send, be liberal in what you accept."
However, I'd like to offer a different perspective that might be worth
considering.

A 2021 IETF draft, "The Harmful Consequences of the Robustness Principle" [2]https://www.ietf.org/archive/id/draft-iab-protocol-maintenance-05.html,
argues that the flexibility advocated by Postel's Law can lead to problems such
as unclear specifications and a multitude of varying implementations. Features
that initially seem helpful can unexpectedly turn into bugs, resulting in
unanticipated consequences and data integrity risks.

Based on the feedback from you and others, I'd like to revise my earlier
proposal. Rather than adding an option to preserve the existing behavior, I now
think it's better to simply report an error in such cases. This approach offers
several benefits: it simplifies the CSV parser, reduces the risk of
misinterpreting data due to malformed input, and prevents the all-too-familiar
situation where users blindly apply an error hint without understanding the
consequences.

Finally, I acknowledge that we can't foresee the number of CSV producers that
produce mid-field quoting, and this change may cause compatibility issues for
some users. However, I consider this an acceptable tradeoff. Users encountering
the error would receive a clear message explaining that mid-field quoting is not
allowed and that they should change their CSV producer's settings to escape
quotes by doubling the quote character. Importantly, this change guarantees that
previously parsed data won't be misinterpreted, as it only enforces stricter
parsing rules.

[1]: https://datatracker.ietf.org/doc/html/rfc761#section-2.10
[2]: https://www.ietf.org/archive/id/draft-iab-protocol-maintenance-05.html

/Joel

On 2023-05-13 Sa 04:20, Joel Jacobson wrote:

On Fri, May 12, 2023, at 21:57, Andrew Dunstan wrote:

Maybe this is unexpected by you, but it's not by me. What other sane
interpretation of that data could there be? And what CSV producer
outputs such horrible content? As you've noted, ours certainly does
not. Our rules are clear: quotes within quotes must be escaped
(default escape is by doubling the quote char). Allowing partial
fields to be quoted was a deliberate decision when CSV parsing was
implemented, because examples have been seen in the wild.

So I don't think our behaviour is broken or needs fixing. As
mentioned by Greg, this is an example of the adage about being
liberal in what you accept.

I understand your position, and your points are indeed in line with the
traditional "Robustness Principle" (aka "Postel's Law") [1] from 1980,
which
suggests "be conservative in what you send, be liberal in what you
accept."
However, I'd like to offer a different perspective that might be worth
considering.

A 2021 IETF draft, "The Harmful Consequences of the Robustness
Principle" [2],
argues that the flexibility advocated by Postel's Law can lead to
problems such
as unclear specifications and a multitude of varying implementations.
Features
that initially seem helpful can unexpectedly turn into bugs, resulting in
unanticipated consequences and data integrity risks.

Based on the feedback from you and others, I'd like to revise my earlier
proposal. Rather than adding an option to preserve the existing
behavior, I now
think it's better to simply report an error in such cases. This
approach offers
several benefits: it simplifies the CSV parser, reduces the risk of
misinterpreting data due to malformed input, and prevents the
all-too-familiar
situation where users blindly apply an error hint without
understanding the
consequences.

Finally, I acknowledge that we can't foresee the number of CSV
producers that
produce mid-field quoting, and this change may cause compatibility
issues for
some users. However, I consider this an acceptable tradeoff. Users
encountering
the error would receive a clear message explaining that mid-field
quoting is not
allowed and that they should change their CSV producer's settings to
escape
quotes by doubling the quote character. Importantly, this change
guarantees that
previously parsed data won't be misinterpreted, as it only enforces
stricter
parsing rules.

[1] https://datatracker.ietf.org/doc/html/rfc761#section-2.10
[2] https://www.ietf.org/archive/id/draft-iab-protocol-maintenance-05.html

I'm pretty reluctant to change something that's been working as designed
for almost 20 years, and about which we have hitherto had zero
complaints that I recall.

I could see an argument for a STRICT mode which would disallow partially
quoted fields, although I'd like some evidence that we're dealing with a
real problem here. Is there really a CSV producer that produces output
like that you showed in your example? And if so has anyone objected to
them about the insanity of that?

cheers

andrew

--
Andrew Dunstan
EDB:https://www.enterprisedb.com

Andrew Dunstan <andrew@dunslane.net> writes:

I could see an argument for a STRICT mode which would disallow partially
quoted fields, although I'd like some evidence that we're dealing with a
real problem here. Is there really a CSV producer that produces output
like that you showed in your example? And if so has anyone objected to
them about the insanity of that?

I think you'd want not just "some evidence" but "compelling evidence".
Any such option is going to add cycles into the low-level input parser
for COPY, which we know is a hot spot and we've expended plenty of
sweat on. Adding a speed penalty that will be paid by the 99.99%
of users who don't have an issue here is going to be a hard sell.

regards, tom lane

On Sat, 13 May 2023 at 09:46, Tom Lane <tgl@sss.pgh.pa.us> wrote:

Andrew Dunstan <andrew@dunslane.net> writes:

I could see an argument for a STRICT mode which would disallow partially
quoted fields, although I'd like some evidence that we're dealing with a
real problem here. Is there really a CSV producer that produces output
like that you showed in your example? And if so has anyone objected to
them about the insanity of that?

I think you'd want not just "some evidence" but "compelling evidence".
Any such option is going to add cycles into the low-level input parser
for COPY, which we know is a hot spot and we've expended plenty of
sweat on. Adding a speed penalty that will be paid by the 99.99%
of users who don't have an issue here is going to be a hard sell.

Well I'm not sure that follows. Joel specifically claimed that an
implementation that didn't accept inputs like this would actually be
simpler and that might mean it would actually be faster.

And I don't think you have to look very hard for inputs like this --
plenty of people generate CSV files from simple templates or script
outputs that don't understand escaping quotation marks at all. Outputs
like that will be fine as long as there's no doublequotes in the
inputs but then one day someone will enter a doublequote in a form
somewhere and blammo.

So I guess the real question is whether accepting inputs with
unescapted quotes and interpreting them the way we do is really the
best interpretation. Is the user best served by a) assuming they
intended to quote part of the field and not quote part of it b) assume
they failed to escape the quotation mark or c) assume something's gone
wrong and the input is entirely untrustworthy.

--
greg

On 2023-05-13 Sa 23:11, Greg Stark wrote:

On Sat, 13 May 2023 at 09:46, Tom Lane<tgl@sss.pgh.pa.us> wrote:

Andrew Dunstan<andrew@dunslane.net> writes:

I could see an argument for a STRICT mode which would disallow partially
quoted fields, although I'd like some evidence that we're dealing with a
real problem here. Is there really a CSV producer that produces output
like that you showed in your example? And if so has anyone objected to
them about the insanity of that?

I think you'd want not just "some evidence" but "compelling evidence".
Any such option is going to add cycles into the low-level input parser
for COPY, which we know is a hot spot and we've expended plenty of
sweat on. Adding a speed penalty that will be paid by the 99.99%
of users who don't have an issue here is going to be a hard sell.

Well I'm not sure that follows. Joel specifically claimed that an
implementation that didn't accept inputs like this would actually be
simpler and that might mean it would actually be faster.

And I don't think you have to look very hard for inputs like this --
plenty of people generate CSV files from simple templates or script
outputs that don't understand escaping quotation marks at all. Outputs
like that will be fine as long as there's no doublequotes in the
inputs but then one day someone will enter a doublequote in a form
somewhere and blammo.

The procedure described is plain wrong, and I don't have too much
sympathy for people who implement it. Parsing CSV files might be a mild
PITN, but constructing them is pretty darn simple. Something like this
perl fragment should do it:

do {
  s/"/""/g;
  $_ = qq{"$_"} if /[",\r\n]/;
} foreach @fields;
print join(',',@fields),"\n";

And if people do follow the method you describe then their input with
unescaped quotes will be rejected 999 times out of 1000. It's only cases
where the field happens to have an even number of embedded quotes, like
Joel's somewhat contrived example, that the input will be accepted.

So I guess the real question is whether accepting inputs with
unescapted quotes and interpreting them the way we do is really the
best interpretation. Is the user best served by a) assuming they
intended to quote part of the field and not quote part of it b) assume
they failed to escape the quotation mark or c) assume something's gone
wrong and the input is entirely untrustworthy.

As I said earlier, I'm quite reluctant to break things that might have
been working happily for people for many years, in order to accommodate
people who can't do the minimum required to produce correct CSVs. I have
no idea how many are relying on it, but I would be slightly surprised if
the number were zero.

cheers

andrew

--
Andrew Dunstan
EDB:https://www.enterprisedb.com

On Sun, May 14, 2023, at 16:58, Andrew Dunstan wrote:

And if people do follow the method you describe then their input with
unescaped quotes will be rejected 999 times out of 1000. It's only cases where
the field happens to have an even number of embedded quotes, like Joel's
somewhat contrived example, that the input will be accepted.

I concur with Andrew that my previous example might've been somewhat
contrived, as it deliberately included two instances of the term "inches".
It's a matter of time before someone submits a review featuring an odd number of
"inches", leading to an error.

Having done some additional digging, I stumbled upon three instances [1]/messages/by-id/1upfg19cru2jigbm553fugj5k6iebtd4ps@4ax.com [2]https://stackoverflow.com/questions/44108286/unterminated-csv-quoted-field-in-postgres [3]https://dba.stackexchange.com/questions/306662/unterminated-csv-quoted-field-when-to-import-csv-data-file-into-postgresql
where users have misidentified their TSV/TEXT files as CSV. In these situations,
users have been shielded from failure due to an imbalance in quotes.

However, in cases where a field is utilized to store text in which the double
quotation mark is rarely used to denote inches, but instead, for quotation
and/or HTML attributes, it's quite feasible that a large amount of
user-generated text could contain balanced quotes. Even more concerning is the
potential for cases where the vast majority of inputs may not even contain
double quotation marks at all. This would effectively render the issue invisible,
even upon manual data inspection.

Here's a problem scenario that I believe is plausible:

1. The user wishes to import a .TXT file into PostgreSQL.

2. The user examines the .TXT file and observes column headers separated by a
delimiter like TAB or semicolon, with subsequent rows of data also separated by
the same delimiter.

3. The user is familiar with "CSV" (412M hits on Google) but not "TSV" (48M hits
on Google), leading to a false assumption that their file is in CSV format.

4. A Google search for "import csv into postgresql" leads the user to a tutorial
titled "Import CSV File Into PostgreSQL Table". An example found therein:

COPY persons(first_name, last_name, dob, email)
FROM 'C:\sampledb\persons.csv'
DELIMITER ','
CSV HEADER;

5. The user, now confident, believes they understand how to import their "CSV"
file.

6. In contrast to the "ERROR: unterminated CSV quoted field" examples below,
this user's .TXT file contains fields with balanced midfield quote-marks:

blog_posts.txt:
id message
1 This is a <b>bold</b> statement

7. The user copies the COPY command from the tutorial and modifies the file path
and delimiter accordingly. The user then concludes that the code is functioning
as expected and proceeds to deploy it.

8, Weeks later, users complain about broken links in their blog posts. Upon
inspection of the blog_posts table, the user identifies an issue:

SELECT * FROM blog_posts;
id | message
----+------------------------------------------------------------------------
1 | This is a <b>bold</b> statement
2 | Check <a href=http://example.com/?param1=Midfield quoting>this</a> out
(2 rows)

One of the users has used balanced quotes for the href attribute, which was
imported successfully but the quotes were stripped, contrary to the intention of
preserving them.

Content of blog_posts.txt:
id message
1 This is a <b>bold</b> statement
2 Check <a href="http://example.com/?param1=Midfield quoting">this</a> out

If we made midfield quoting a CSV error, those users who are currently mistaken
about their TSV/TEXT files being CSV while also having balanced quotes in their
data, would encounter an error rather than a silent failure, which I believe
would be an enhancement.

/Joel

[1]: /messages/by-id/1upfg19cru2jigbm553fugj5k6iebtd4ps@4ax.com
[2]: https://stackoverflow.com/questions/44108286/unterminated-csv-quoted-field-in-postgres
[3]: https://dba.stackexchange.com/questions/306662/unterminated-csv-quoted-field-when-to-import-csv-data-file-into-postgresql

On Tue, May 16, 2023, at 13:43, Joel Jacobson wrote:

If we made midfield quoting a CSV error, those users who are currently mistaken
about their TSV/TEXT files being CSV while also having balanced quotes in their
data, would encounter an error rather than a silent failure, which I believe
would be an enhancement.

Furthermore, I think it could be beneficial to add a HINT message for all type
of CSV/TEXT parsing errors, since the precise ERROR messages might just cause
the user to tinker with the options until it works, instead of carefully reading
through the documentation on the various formats.

Perhaps something like this:

HINT: Are you sure the FORMAT matches your input?

Also, the COPY documentation says nothing about TSV, and I know TEXT isn't
exactly TSV, but it's at least much more TSV than CSV, so maybe we should
describe the differences, such as \N. I think the best advise to users would be
to avoid exporting to .TSV and use .CSV instead, since I've noticed e.g.
Google Sheets to replace newlines in fields with blank space when
exporting .TSV, which effectively destroys data.

The first search results for "postgresql tsv" on Google link to postgresql.org
pages, but the COPY docs are not one of them unfortunately.

The first relevant hit is this one:

"Importing a TSV File into Postgres | by Riley Wong" [1]https://medium.com/@rlwong2/importing-a-tsv-file-into-postgres-364572a004bf

Sadly, this author has also misunderstood how to properly import a .TSV file,
he got it all wrong, and doesn't understand or at least doesn't mention there
are more differences than just the delimiter:

COPY listings
FROM '/home/ec2-user/list.tsv'
DELIMITER E'\t'
CSV HEADER;

I must confess I have used PostgreSQL for over two decades without having really
understood the detailed differences between TEXT and CSV, until recently.

[1]: https://medium.com/@rlwong2/importing-a-tsv-file-into-postgres-364572a004bf

On 2023-05-16 Tu 13:15, Joel Jacobson wrote:

On Tue, May 16, 2023, at 13:43, Joel Jacobson wrote:

If we made midfield quoting a CSV error, those users who are

currently mistaken

about their TSV/TEXT files being CSV while also having balanced

quotes in their

data, would encounter an error rather than a silent failure, which I

believe

would be an enhancement.

Furthermore, I think it could be beneficial to add a HINT message for
all type
of CSV/TEXT parsing errors, since the precise ERROR messages might
just cause
the user to tinker with the options until it works, instead of
carefully reading
through the documentation on the various formats.

Perhaps something like this:

HINT: Are you sure the FORMAT matches your input?

Also, the COPY documentation says nothing about TSV, and I know TEXT isn't
exactly TSV, but it's at least much more TSV than CSV, so maybe we should
describe the differences, such as \N. I think the best advise to users
would be
to avoid exporting to .TSV and use .CSV instead, since I've noticed e.g.
Google Sheets to replace newlines in fields with blank space when
exporting .TSV, which effectively destroys data.

The first search results for "postgresql tsv" on Google link to
postgresql.org
pages, but the COPY docs are not one of them unfortunately.

The first relevant hit is this one:

"Importing a TSV File into Postgres | by Riley Wong" [1]

Sadly, this author has also misunderstood how to properly import a
.TSV file,
he got it all wrong, and doesn't understand or at least doesn't
mention there
are more differences than just the delimiter:

COPY listings
FROM '/home/ec2-user/list.tsv'
DELIMITER E'\t'
CSV HEADER;

I must confess I have used PostgreSQL for over two decades without
having really
understood the detailed differences between TEXT and CSV, until recently.

[1]
https://medium.com/@rlwong2/importing-a-tsv-file-into-postgres-364572a004bf

You can use CSV mode pretty reliably for TSV files. The trick is to use
a quoting char that shouldn't appear, such as E'\x01' as well as setting
the delimiter to E'\t'. Yes, it's far from obvious.

cheers

andrew

--
Andrew Dunstan
EDB:https://www.enterprisedb.com

On Wed, May 17, 2023, at 19:42, Andrew Dunstan wrote:

You can use CSV mode pretty reliably for TSV files. The trick is to use a
quoting char that shouldn't appear, such as E'\x01' as well as setting the
delimiter to E'\t'. Yes, it's far from obvious.

I've been using that trick myself many times in the past, but thanks to this
deep-dive into this topic, it looks to me like TEXT would be a better format
fit when dealing with unquoted TSV files, or?

OTOH, one would then need to inspect the TSV file doesn't contain \. on an empty
line...

I was about to suggest we perhaps should consider adding a TSV format, that
is like TEXT excluding the PostgreSQL specific things like \. and \N,
but then I tested exporting TSV from Numbers on Mac and Google Sheets,
and I can see there are incompatible differences. Numbers quote fields
that contain double-quote marks, while Google Sheets doesn't.
None of them (unsurpringly) uses midfield quoting though.

Anyone using Excel that could try exporting the following example as CSV/TSV?

CREATE TABLE t (a text, b text, c text, d text);
INSERT INTO t (a, b, c, d)
VALUES ('unquoted','a "quoted" string', 'field, with a comma', E'field\t with a tab');

I agree with you that it's unwise to change something that's been working
great for such a long time, and I agree CSV-files are probably not a problem
per se, but I think you will agree with me TSV-files is a different story,
from a user-friendliness and correctness perspective. Sure, we could just say
"Don't use TSV! Use CSV instead!" in the docs, that would be an improvement
I think, but there is currently nothing on "TSV" in the docs, so users will
google and find all these broken dangerous suggestions on work-arounds.

Thoughts?

/Joel

On Wed, May 17, 2023 at 5:47 PM Joel Jacobson <joel@compiler.org> wrote:

On Wed, May 17, 2023, at 19:42, Andrew Dunstan wrote:

You can use CSV mode pretty reliably for TSV files. The trick is to use a
quoting char that shouldn't appear, such as E'\x01' as well as setting

the

delimiter to E'\t'. Yes, it's far from obvious.

I've been using that trick myself many times in the past, but thanks to
this
deep-dive into this topic, it looks to me like TEXT would be a better
format
fit when dealing with unquoted TSV files, or?

OTOH, one would then need to inspect the TSV file doesn't contain \. on an
empty
line...

I was about to suggest we perhaps should consider adding a TSV format, that
is like TEXT excluding the PostgreSQL specific things like \. and \N,
but then I tested exporting TSV from Numbers on Mac and Google Sheets,
and I can see there are incompatible differences. Numbers quote fields
that contain double-quote marks, while Google Sheets doesn't.
None of them (unsurpringly) uses midfield quoting though.

Anyone using Excel that could try exporting the following example as
CSV/TSV?

CREATE TABLE t (a text, b text, c text, d text);
INSERT INTO t (a, b, c, d)
VALUES ('unquoted','a "quoted" string', 'field, with a comma', E'field\t
with a tab');

Here you go. Not horrible handling. (I use DataGrip so I saved it from
there directly as TSV,
just for an extra datapoint).

FWIW, if you copy/paste in windows, the data, the field with the tab gets
split into another column in Excel.
But saving it as a file, and opening it.
Saving it as XLSX, and then having Excel save it as a TSV (versus opening a
text file, and saving it back)

Kirk...

On Thu, May 18, 2023, at 00:18, Kirk Wolak wrote:

Here you go. Not horrible handling. (I use DataGrip so I saved it from there
directly as TSV, just for an extra datapoint).

FWIW, if you copy/paste in windows, the data, the field with the tab gets
split into another column in Excel. But saving it as a file, and opening it.
Saving it as XLSX, and then having Excel save it as a TSV (versus opening a
text file, and saving it back)

Very useful, thanks.

Interesting, DataGrip contrary to Excel doesn't quote fields with commas in TSV.
All the DataGrip/Excel TSV variants uses quoting when necessary,
contrary to Google Sheets's TSV-format, that doesn't quote fields at all.

DataGrip/Excel terminate also the last record with newline,
while Google Sheets omit the newline for the last record,
(which is bad, since then a streaming reader wouldn't know
if the last record is completed or not.)

This makes me think we probably shouldn't add a new TSV format,
since there is no consistency between vendors.
It's impossible to deduce with certainty if a TSV-field that
begins with a double quotation mark is quoted or unquoted.

Two alternative ideas:

1. How about adding a `WITHOUT QUOTE` or `QUOTE NONE` option in conjunction
with `COPY ... WITH CSV`?

Internally, it would just set

quotec = '\0';`

so it would't affect performance at all.

2. How about adding a note on the complexities of dealing with TSV files in the
COPY documentation?

/Joel

On Thu, May 18, 2023, at 08:00, Joel Jacobson wrote:

1. How about adding a `WITHOUT QUOTE` or `QUOTE NONE` option in conjunction
with `COPY ... WITH CSV`?

More ideas:
[ QUOTE 'quote_character' | UNQUOTED ]
or
[ QUOTE 'quote_character' | NO_QUOTE ]

Thinking about it, I recall another hack;
specifying a non-existing char as the delimiter to force the entire line into a
single column table. For that use-case, we could also provide an option that
would internally set:

delimc = '\0';

How about:

[DELIMITER 'delimiter_character' | UNDELIMITED ]
or
[DELIMITER 'delimiter_character' | NO_DELIMITER ]
or it should be more use-case-based and intuitive:
[DELIMITER 'delimiter_character' | WHOLE_LINE_AS_RECORD ]

/Joel

čt 18. 5. 2023 v 8:01 odesílatel Joel Jacobson <joel@compiler.org> napsal:

On Thu, May 18, 2023, at 00:18, Kirk Wolak wrote:

Here you go. Not horrible handling. (I use DataGrip so I saved it from

there

directly as TSV, just for an extra datapoint).

FWIW, if you copy/paste in windows, the data, the field with the tab gets
split into another column in Excel. But saving it as a file, and opening

it.

Saving it as XLSX, and then having Excel save it as a TSV (versus

opening a

text file, and saving it back)

Very useful, thanks.

Interesting, DataGrip contrary to Excel doesn't quote fields with commas
in TSV.
All the DataGrip/Excel TSV variants uses quoting when necessary,
contrary to Google Sheets's TSV-format, that doesn't quote fields at all.

Maybe there is another third implementation in Libre Office.

Generally TSV is not well specified, and then the implementations are not
consistent.

On Thu, May 18, 2023, at 08:35, Pavel Stehule wrote:

Maybe there is another third implementation in Libre Office.

Generally TSV is not well specified, and then the implementations are not consistent.

Thanks Pavel, that was a very interesting case indeed:

Libre Office (tested on Mac) doesn't have a separate TSV format,
but its CSV format allows specifying custom "Field delimiter" and
"String delimiter".

How peculiar, in Libre Office, when trying to write double quotation marks
(using Shift+2 on my keyboard) you actually don't get the normal double
quotation marks, but some special type of Unicode-quoting,
e2 80 9c ("LEFT DOUBLE QUOTATION MARK") and
e2 80 9d ("RIGHT DOUBLE QUOTATION MARK"),
and in the .CSV file you get the normal double quotation marks as
"String delimiter":

a,b,c,d,e
unquoted,“this field is quoted”,this “word” is quoted,"field with , comma",field with tab

So, my "this field is quoted" experiment was exported unquoted since their
quotation marks don't need to be quoted.

On 2023-05-18 Th 02:19, Joel Jacobson wrote:

On Thu, May 18, 2023, at 08:00, Joel Jacobson wrote:

1. How about adding a `WITHOUT QUOTE` or `QUOTE NONE` option in

conjunction

with `COPY ... WITH CSV`?

More ideas:
[ QUOTE 'quote_character' | UNQUOTED ]
or
[ QUOTE 'quote_character' | NO_QUOTE ]

Thinking about it, I recall another hack;
specifying a non-existing char as the delimiter to force the entire
line into a
single column table. For that use-case, we could also provide an
option that
would internally set:

    delimc = '\0';

How about:

[DELIMITER 'delimiter_character' | UNDELIMITED ]
or
[DELIMITER 'delimiter_character' | NO_DELIMITER ]
or it should be more use-case-based and intuitive:
[DELIMITER 'delimiter_character' | WHOLE_LINE_AS_RECORD ]

QUOTE NONE and DELIMITER NONE should work fine. NONE is already a
keyword, so the disturbance should be minimal.

cheers

andrew

--
Andrew Dunstan
EDB:https://www.enterprisedb.com

Joel Jacobson wrote:

I've been using that trick myself many times in the past, but thanks to this
deep-dive into this topic, it looks to me like TEXT would be a better format
fit when dealing with unquoted TSV files, or?

OTOH, one would then need to inspect the TSV file doesn't contain \. on an
empty line...

Note that this is the case for valid CSV contents, since backslash-dot
on a line by itself is both an end-of-data marker for COPY FROM and a
valid CSV line.
Having this line in the data results in either an error or having the
rest of the data silently discarded, depending on the context. There
is some previous discussion about this in [1]/messages/by-id/10e3eff6-eb04-4b3f-aeb4-b920192b977a@manitou-mail.org.
Since the TEXT format doesn't have this kind of problem, one solution
is to filter the data through PROGRAM with an [untrusted CSV]->TEXT
filter. This is to be preferred over direct CSV loading when
strictness or robustness are more important than convenience.

[1]: /messages/by-id/10e3eff6-eb04-4b3f-aeb4-b920192b977a@manitou-mail.org
/messages/by-id/10e3eff6-eb04-4b3f-aeb4-b920192b977a@manitou-mail.org

Best regards,
--
Daniel Vérité
https://postgresql.verite.pro/
Twitter: @DanielVerite