Re: [sqlalchemy] MSSQL String columns often incorrectly encoded as NVARCHAR
Thanks. I’ll pursue those avenues. Cheers, Ian 2019년 4월 1일 (월) 11:30, Mike Bayer 님이 작성: > On Sun, Mar 31, 2019 at 10:12 PM Ian Wagner > wrote: > > > > > > My suggestion would be a pymssql dialect-level patch to send bytestrings > for String columns, and of course continue passing str/unicode for Unicode > columns. I'm on the mailing list looking for help with why my solution > doesn't work as intended with enums (see my GitHub repo). For the moment, > I've actually just implemented this column myself as shown, and replaced > all String columns with it (rather than "replacing" String using colspecs), > and banned all devs on our project from using the regular String column > type until it's fixed. > > Please just use a TypeDecorator for all your String columns where this > issue is apparent: > > class UTF8String(TypeDecorator): > impl = String > > def process_bind_param(self, value, dialect): > if value is not None: > value = value.encode(dialect.encoding) > return value > > next, the issue should be reported to pymssql, where they should > likely include options to modify this behavior: > > https://github.com/pymssql/pymssql > > next, I need this to be confirmed as an issue for pyodbc.As my > understanding is that Microsoft is funding Pyodbc's development I'd > like to see what their approach to this issue is. > > *if* it is truly the canonical solution that applications that code > against these DBAPIs *must* send byte arrays in order to avoid > crushing performance costs, the drivers need to make that clear. > then we can look into introducing either flags or permanent behavior > such that we encode all Python unicode objects for a *non unicode* > String datatype under the MSSQL dialects. > > > > > > >> Also note pymssql is not well maintained right now due to lack of > >> funding > > > > > > Noted. We'll look into switching drivers yet again, but the official > driver lacked several features the last time we evaluated it (I think > stored proc output parameters were not supported, and it would require > quite a few syntax changes in areas where we have to write raw SQL). > > > > Cheers, > > Ian > > > > On Friday, March 29, 2019 at 11:10:21 PM UTC+9, Mike Bayer wrote: > >> > >> Also note pymssql is not well maintained right now due to lack of > >> funding, please confirm you reproduce your performance concerns using > >> PyODBC with Microsofts ODBC drivers ? That should be considered to be > >> the canonically supported driver right now, works on all platforms > >> very well now. > >> > >> On Fri, Mar 29, 2019 at 10:07 AM Mike Bayer > wrote: > >> > > >> > OK so I saw that the "N" prefix is not generated with your test case > >> > either, so I re-read your email. Can you clarify what you mean by > >> > "always encoded as NVARCHAR"? are you referring to the simple fact > >> > that a Python string object is passed to the driver, and that the > >> > driver is behind the scenes applying the "N" in any case or is > >> > otherwise binding it as unicode in such a way that performance is > >> > impacted? SQLAlchemy for many years passed bytestrings to drivers > >> > like pyodbc because they would simply crash if you passed them a > >> > unicode object, but once they supported it, SQLAlchemy was eager to > >> > get out of the business of doing this encoding.In 1.3 we've just > >> > deprecated all the flags that allow it to do this > >> > (convert_unicode=True).Using that flag would be your quickest way > >> > to get it back for now but we'd have to develop an all new behavior > >> > for 1.4 if we are to start encoding these binds again, however current > >> > behavior has been this way for many years and this is the first it's > >> > being reported in this way. I would want to look into driver > >> > configuration options for this as well. > >> > > >> > > >> > > >> > > >> > On Fri, Mar 29, 2019 at 9:56 AM Mike Bayer > wrote: > >> > > > >> > > On Fri, Mar 29, 2019 at 6:20 AM Ian Wagner > wrote: > >> > > > > >> > > > Hello all, > >> > > > > >> > > > I'm trying to get to the bottom of an issue in which Python 3 > (unicode by definition) strings are always encoded as NVARCHAR for at least > two backends (pymssql and pyodbc). Using bytstring
Re: [sqlalchemy] MSSQL String columns often incorrectly encoded as NVARCHAR
" in any case or is
> > otherwise binding it as unicode in such a way that performance is
> > impacted? SQLAlchemy for many years passed bytestrings to drivers
> > like pyodbc because they would simply crash if you passed them a
> > unicode object, but once they supported it, SQLAlchemy was eager to
> > get out of the business of doing this encoding.In 1.3 we've just
> > deprecated all the flags that allow it to do this
> > (convert_unicode=True).Using that flag would be your quickest way
> > to get it back for now but we'd have to develop an all new behavior
> > for 1.4 if we are to start encoding these binds again, however current
> > behavior has been this way for many years and this is the first it's
> > being reported in this way. I would want to look into driver
> > configuration options for this as well.
> >
> >
> >
> >
> > On Fri, Mar 29, 2019 at 9:56 AM Mike Bayer > wrote:
> > >
> > > On Fri, Mar 29, 2019 at 6:20 AM Ian Wagner > wrote:
> > > >
> > > > Hello all,
> > > >
> > > > I'm trying to get to the bottom of an issue in which Python 3
> (unicode by definition) strings are always encoded as NVARCHAR for at least
> two backends (pymssql and pyodbc). Using bytstrings as comparison arguments
> (for example Table.column == value.encode('utf-8')) sends a regular string
> literal as expected, but regular strings are encoded as NVARCHAR literals.
> > > >
> > > > This behavior is fairly logical at the underlying driver (pymssql or
> pyodbc) level, which is why I'm posting here. I believe the the use of a
> String column (as opposed to a Unicode column) type should not pass an
> NVARCHAR literal. Doing so has disastrous performance implications, as SQL
> Server ends up casting the whole column up. This will wreak havoc when
> regularly dealing with large-ish tables (1.7 million rows or so in our
> case).
> > > >
> > > > I have previously posted with a LOT more details on StackOverflow (
> https://stackoverflow.com/questions/55098426/strings-used-in-query-always-sent-with-nvarchar-syntax-even-if-the-underlying-c).
>
> I also have an MCVE over on GitHub (
> https://github.com/ianthetechie/pymssql_sqlalchemy_55098426).
> > > >
> > > > In my MCVE, I outline a possible approach for fixing this, but it
> appears to have some problems. I'm posting here asking for feedback on
> what's wrong with my approach, and what would be the best way to go about
> getting this fixed.
> > >
> > > seems like we will need some documentation for this as it is confusing
> > > a lot of people. The issue that introduced this behavior is
> > > https://github.com/sqlalchemy/sqlalchemy/issues/4442 and then that
> > > same user felt it was happening too often in
> > > https://github.com/sqlalchemy/sqlalchemy/issues/4561, however I
> > > clarified that the N prefix only generates if there is no other
> > > context to determine that this is not a non-unicode context.
> > >
> > > the NVARCHAR thing should not happen if you are comparing to a
> > > non-NVARCHAR column. it only occurs when there is no other context
> > > that SQLAlchemy can determine the correct datatype for the Unicode
> > > object being passed.
> > >
> > > However, the example case you have on github there seems to be using a
> > > unicode in a VARCHAR comparison so should not see the N. If it does,
> > > it's a bug. I will try your test case now.
> > >
> > > In the future, please go straight to the SQLAlchemy github issue
> > > tracker with a succinct test case, as this N thing is obviously still
> > > ongoing.
> > >
> > >
> > >
> > > >
> > > > Thanks!
> > > > Ian
> > > >
> > > > --
> > > > SQLAlchemy -
> > > > The Python SQL Toolkit and Object Relational Mapper
> > > >
> > > > http://www.sqlalchemy.org/
> > > >
> > > > To post example code, please provide an MCVE: Minimal, Complete, and
> Verifiable Example. See http://stackoverflow.com/help/mcve for a full
> description.
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--
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http://www.sqlalchemy.org/
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Example. See http://stackoverflow.com/help/mcve for a full description.
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[sqlalchemy] MSSQL String columns often incorrectly encoded as NVARCHAR
Hello all,
I'm trying to get to the bottom of an issue in which Python 3 (unicode by
definition) strings are always encoded as NVARCHAR for at least two
backends (pymssql and pyodbc). Using bytstrings as comparison arguments
(for example Table.column == value.encode('utf-8')) sends a regular string
literal as expected, but regular strings are encoded as NVARCHAR literals.
This behavior is fairly logical at the underlying driver (pymssql or
pyodbc) level, which is why I'm posting here. I believe the the use of a
String column (as opposed to a Unicode column) type should not pass an
NVARCHAR literal. Doing so has disastrous performance implications, as SQL
Server ends up casting the whole column up. This will wreak havoc when
regularly dealing with large-ish tables (1.7 million rows or so in our
case).
I have previously posted with a LOT more details on StackOverflow
(https://stackoverflow.com/questions/55098426/strings-used-in-query-always-sent-with-nvarchar-syntax-even-if-the-underlying-c).
I also have an MCVE over on GitHub
(https://github.com/ianthetechie/pymssql_sqlalchemy_55098426).
In my MCVE, I outline a possible approach for fixing this, but it appears
to have some problems. I'm posting here asking for feedback on what's wrong
with my approach, and what would be the best way to go about getting this
fixed.
Thanks!
Ian
--
SQLAlchemy -
The Python SQL Toolkit and Object Relational Mapper
http://www.sqlalchemy.org/
To post example code, please provide an MCVE: Minimal, Complete, and Verifiable
Example. See http://stackoverflow.com/help/mcve for a full description.
---
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