Add query transformation callback feature

[soci]

Add query transformation callback feature

[mloskot]

I'm going to implement the option 2. for SOCI 3.2.0

[vadz]

It's not really an objection but I have a feeling that working with SQL at string level is not really the right thing to do. One thing that bothers me is that the examples above break down for the queries which already include a WHERE clause. Another one is that "transformation" can be anything at all (including absence of changes, the mechanism for which I'd consider using this would be for logging all SQL queries executed in my code).

So if we just want to allow defining extra WHERE clauses for the queries which don't necessarily include them, it'd be IMO better to provide some soci::session::set_projection() which could, at least in principle, be implemented correctly even for the queries which already have WHERE clauses and which is also more clear.

But maybe I just don't see the intended use case for this feature. For me the idea looks dangerous (my first thought was: "spooky action at a distance", this is going to be fun to debug) rather than interesting...

[mloskot]

@vadz The idea is not to simply allow extra WHERE (or any other SQL clause), but to enable users to perform some action on query string just before it's executed. In fact, it may be used as part of security measures, a sanitisation checking for any unverified input (SQL injection comes to mind). My examples are just poor and don't back up the idea well.

Certainly, this kind of "working with SQL at string level" is an advanced feature.

[vnaydionov]

I can imagine why it might be useful.
For example, in Oracle one can insert specially formatted comment just after SELECT keyword to force certain behaviour for the query optimizer.
In particular, the following directive enforces using rule-based optimizer rather than cost-based one.
SELECT /+RULE/ A, B FROM C

Also, here is an example of more specialized interface, allowing for altering queries
http://docs.sqlalchemy.org/en/rel_0_7/core/expression_api.html#sqlalchemy.sql.expression.Select.prefix_with

[mloskot]

@vnaydionov I didn't know about this feature. Nice one.

[vadz]

On Sun, 24 Feb 2013 11:21:48 -0800 Viacheslav Naydenov [email protected] wrote:

VN> I can imagine why it might be useful.
VN> For example, in Oracle one can insert specially formatted comment just after SELECT keyword to force certain behaviour for the query optimizer.
VN> In particular, the following directive enforces using rule-based optimizer rather than cost-based one.
VN> SELECT /+RULE/ A, B FROM C
VN>
VN> Also, here is an example of more specialized interface, allowing for altering queries
VN> http://docs.sqlalchemy.org/en/rel_0_7/core/expression_api.html#sqlalchemy.sql.expression.Select.prefix_with

Interesting, I didn't know about either of these uses. Perhaps a mention
of them could be added to the documentation for this feature to make it
more clear what kind of things can be done with it.

VZ

[mloskot]

@vadz good idea, I'll add a note about this usage, as part of #85

[dstocking-ext]

Is there any way that this could be changed so that it gives the actual sql statement object? If I want to add more to a where clause or what have you I cannot use a soci::use(). Idk if a soci::use sanatizes the input, but still would like it all in one spot.