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 <[email protected]> wrote: > > On Fri, Mar 29, 2019 at 6:20 AM Ian Wagner <[email protected]> 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. > > --- > > You received this message because you are subscribed to the Google Groups > > "sqlalchemy" group. > > To unsubscribe from this group and stop receiving emails from it, send an > > email to [email protected]. > > To post to this group, send email to [email protected]. > > Visit this group at https://groups.google.com/group/sqlalchemy. > > For more options, visit https://groups.google.com/d/optout. -- 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. --- You received this message because you are subscribed to the Google Groups "sqlalchemy" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send email to [email protected]. Visit this group at https://groups.google.com/group/sqlalchemy. For more options, visit https://groups.google.com/d/optout.
