QSqlQuery::size() not returning actual number?

davecotter

sorry, i guess i should have made my question more generic than "how many rows will be returned from this specific statement". what i should have asked is:

if i've already performed some statement, and i only have access to the QSqlQuery at this point and no longer know what the actual statement was, how do i ask how many rows that returns? I want it to work for any statement, not just one that asks if a table exists. is my solution just grossly inefficient? is there a better way than this:

  			i_qquery.last();
  	
  			sizeI = i_qquery.at();
  			
  			if (sizeI >= 0) {
  				resultI = sizeI + 1;
  			}

Christian Ehrlicher

As @SGaist already mentioned this only works (and unrelated from Qt) when the underlying database supports this. You can check it with QSqlDriver::hasFeature ...

davecotter

yes we have determined that "size()" doesn't work on SQLite, i understand that.

my question has evolved into: is the way i'm doing it now a bad / terribly inefficient way to do it?
for reference, this is what i'm now doing:

i_qquery.last();
		
sizeI = i_qquery.at();
				
if (sizeI >= 0) {
	resultI = sizeI + 1;
}

thanks

Christian Ehrlicher

It's not very efficient but when you really need the result size for whatever reason you've no other chance.

Kent-Dorfman

@davecotter said in QSqlQuery::size() not returning actual number?:

yes we have determined that "size()" doesn't work on SQLite, i understand that.

my question has evolved into: is the way i'm doing it now a bad / terribly inefficient way to do it?

That's already been answered as well...

JonB

@davecotter said in QSqlQuery::size() not returning actual number?:

@JonB I’m not sure I understand your reply, I already am using that select statement. My q is: what is the proper way to ask how many rows were returned?

No, you must look at what I wrote: it's SELECT COUNT(*) instead of SELECT *. And you retrieve that single result number as the first column in the one & only row returned as the result set, and you must know how to read stuff out of the result set else you couldn't be using queries for much.

Let's just summarise something else:

if i've already performed some statement, and i only have access to the QSqlQuery at this point and no longer know what the actual statement was, how do i ask how many rows that returns? I want it to work for any statement, not just one that asks if a table exists.

Yes, if you have previously executed an arbitrary query and now wish to know how many rows it returned you have no choice but to use your method.

is my solution just grossly inefficient? is there a better way than this:

Well, yes, it is inefficient (though unavoidable if you want the apples for other purposes too). Think of this: the SQL server has a bunch of apples. You ask the server to send you all the apples, and you sit & count how many have arrived to get the answer. (It's not the counting per se which is slow, it's the having to send & receive all the apples in order to count them which is bad.) It's not perfect, is it? Let's hope it has not sent you a lot of apples to count!

A much more efficient way is to ask the server to do the counting at its end, not send all the apples, but send a single number saying how many there are. That is what SELECT COUNT(...) does. It's great for getting a quick total of the apples, but quite useless if you want to look at each one for some other reason.

davecotter

see @JonB 's answer above! thanks!

kshegunov

@JonB said in QSqlQuery::size() not returning actual number?:

It's great for getting a quick total of the apples

Yes, with some exceptions where a "quick total" isn't exactly quick ... ;)

JonB

@kshegunov
That's unfair! :) Give me any situation where COUNT() at the server is not vastly quicker than passing row sets to client to count? Give me any situation where COUNT() at server is not vastly quicker than code at server to iteratively count rows? Care to comment on whether SELECT COUNT(*) FROM table, with no WHERE condition like OP's example, allows provider to use huge optimization like it has already maintained total table row count and just returns that? Look, there's "quick" and there's "quick", this one is "quick" but I did not say "instantaneous" or "like lightning" :)

kshegunov

@JonB said in QSqlQuery::size() not returning actual number?:

Give me any situation where COUNT() at the server is not vastly quicker than passing row sets to client to count?

That'd be all the situations where the server is running a InnoDB engine, for example, and the table is much larger than the resultset.

Give me any situation where COUNT() at server is not vastly quicker than code at server to iteratively count rows?

No such situation exists, that's why aggregates exist. Unless you have a stored procedure that does something else, that is, then the counting can be a "byproduct".

Care to comment on whether SELECT COUNT(*) FROM table, with no WHERE condition like OP's example, allows provider to use huge optimization like it has already maintained total table row count and just returns that?

See above. InnoDB doesn't have the real row count at hand. It can give you an approximate amount of rows based on the allocated pages quickly, but for a real row count time of execution is significant, compared to some other engines like (my)ISAM. Anyway, it's more complicated than to claim speed directly.

Look, there's "quick" and there's "quick"

Yes, we agree on that one.

JonB

@kshegunov said in QSqlQuery::size() not returning actual number?:

Give me any situation where COUNT() at the server is not vastly quicker than passing row sets to client to count?

That'd be all the situations where the server is running a InnoDB engine, for example, and the table is much larger than the resultset.

I admit I know nothing about InnoDB [I use MySQL, so I guess I use that?], so your points are fair enough. (For the avoidance of doubt, I am talking about where there is a database "server" involved.) But I don't get this one at all? How can it possibly be quicker to pass whatever all the results are (with or without a condition) to the client to count than to count them at the server side? If you're going to go:

SELECT * FROM table WHERE smaller-resultset-condition

and then send to count them at the client, why is that quicker than

SELECT COUNT(*) FROM table WHERE smaller-resultset-condition

?

kshegunov

@JonB said in QSqlQuery::size() not returning actual number?:

If you're going to go

Because this is not what you usually do when dealing with mid-size to large datasets.
Say for the sake of argument you count the number of rows to provide pagination of some sort, which is a typical case. You count, init the pages' count, current page number or w/e and then retrieve only a single small result set from the table. Right, so you count all relevant records just for the simple paging.
Now, this is okay most of the time, at the end of the day that's why you have a database server (or engine, or w/e) - to provide storage. It gets a bit more involved when you have concurrent transactions over complex queries, however; i.e. you have snapshots all over the place.
Say you count the rows of a query that has a nested query and altogether them queries span 8-10 tables over some JOINs. The count can get real expensive in these cases, especially if you consider why you do it to begin with - to fetch some meta information about the dataset and just provide some small part of it afterwards as actual data. That's what I referred to in my tease.

fanxinglanyu

I think you can use

 if(query.next()){//judge if has data
                    query.seek(-1);
}