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QSqlQuery::size() not returning actual number?

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  • JonBJ JonB

    @davecotter
    As @SGaist has just posted, size() returns -1 if the driver does not support returning the count of the result set.

    Assuming that's the case,

    then i ask how many rows i got back (i want to know if this table exists)

    "SELECT COUNT(*) FROM sqlite_master WHERE type = 'table' AND name = '[tablename]'"
    

    will just return one row containing the number you want (I would assume either 0 or 1 from your query), you might prefer to use that?

    D Offline
    D Offline
    davecotter
    wrote on last edited by
    #5

    @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?

    JonBJ 1 Reply Last reply
    0
    • Kent-DorfmanK Offline
      Kent-DorfmanK Offline
      Kent-Dorfman
      wrote on last edited by Kent-Dorfman
      #6

      As Gaist mentioned, getting the number of rows from a query response is a very haphazard process when you try to implement it for any ole generic database. MANY DBC backends cache returned rows in manageable groups and use something called a cursor to traverse through them. It is possible, and probable, that the real number of rows returned is far more than is in the currently cached response.

      I recommend you used the count() SQL aggregate function to find out how many rows would be returned from your query. It is always guaranteed to return a 1 row result. Looks like JonB also recommended this.

      Worst case is that you have to do two queries. First one to find out the size of your result set.

      D 1 Reply Last reply
      3
      • Kent-DorfmanK Kent-Dorfman

        As Gaist mentioned, getting the number of rows from a query response is a very haphazard process when you try to implement it for any ole generic database. MANY DBC backends cache returned rows in manageable groups and use something called a cursor to traverse through them. It is possible, and probable, that the real number of rows returned is far more than is in the currently cached response.

        I recommend you used the count() SQL aggregate function to find out how many rows would be returned from your query. It is always guaranteed to return a 1 row result. Looks like JonB also recommended this.

        Worst case is that you have to do two queries. First one to find out the size of your result set.

        D Offline
        D Offline
        davecotter
        wrote on last edited by
        #7

        @Kent-Dorfman sorry if I wasn’t clear at the start, this is only SQLite and will never change

        Kent-DorfmanK 1 Reply Last reply
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        • D davecotter

          @Kent-Dorfman sorry if I wasn’t clear at the start, this is only SQLite and will never change

          Kent-DorfmanK Offline
          Kent-DorfmanK Offline
          Kent-Dorfman
          wrote on last edited by
          #8

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

          @Kent-Dorfman sorry if I wasn’t clear at the start, this is only SQLite and will never change

          doesn't matter...using the count() method is the more universal way of doing it.

          D 1 Reply Last reply
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          • Kent-DorfmanK Kent-Dorfman

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

            @Kent-Dorfman sorry if I wasn’t clear at the start, this is only SQLite and will never change

            doesn't matter...using the count() method is the more universal way of doing it.

            D Offline
            D Offline
            davecotter
            wrote on last edited by
            #9

            @Kent-Dorfman I’m not at all versed at this: is there example code showing how to use count()?

            1 Reply Last reply
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            • Kent-DorfmanK Offline
              Kent-DorfmanK Offline
              Kent-Dorfman
              wrote on last edited by Kent-Dorfman
              #10

              yes, plenty, but I have no resources off the top of my head. look at any Qt SQL examples you can find. The key thing is that your quere should be:

              select count(*) as cnt from sqlite_master WHERE type = 'table' AND name = '[tablename]'
              

              and grab the cnt field from the response

              D 1 Reply Last reply
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              • Kent-DorfmanK Kent-Dorfman

                yes, plenty, but I have no resources off the top of my head. look at any Qt SQL examples you can find. The key thing is that your quere should be:

                select count(*) as cnt from sqlite_master WHERE type = 'table' AND name = '[tablename]'
                

                and grab the cnt field from the response

                D Offline
                D Offline
                davecotter
                wrote on last edited by
                #11

                @Kent-Dorfman I’m trying to understand “grab the field from the response” ...

                1 Reply Last reply
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                • Kent-DorfmanK Offline
                  Kent-DorfmanK Offline
                  Kent-Dorfman
                  wrote on last edited by
                  #12

                  a single field row is returned (called cnt). look at the examples and find one where you get the value of a returned field. there are a few ways to do this.

                  Executing your QSqlQuery will return a QResultSet object. The field can be accessed using boundValue(0) method of QResultSet. Ya gotta do some googling and research if you want any more detail than that.

                  1 Reply Last reply
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                  • mrjjM Offline
                    mrjjM Offline
                    mrjj
                    Lifetime Qt Champion
                    wrote on last edited by mrjj
                    #13

                    hi
                    One way could be. ( note my table is called person. change to yours)

                    QSqlQuery query("select count(*) as cnt from sqlite_master where type='table' and name='person'");
                    if (query.next())
                        qDebug() << query.value(0).toInt();
                    

                    However, in org post you said (i want to know if this table exists)
                    so just as a note
                    using SQLite version 3.3+
                    you can say
                    "create table if not exists TableName ..:"
                    if you want to check if table exits and create it if not.
                    Might not be your use case, but its good to know.

                    1 Reply Last reply
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                    • D Offline
                      D Offline
                      davecotter
                      wrote on last edited by davecotter
                      #14

                      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;
                        			}
                      
                      1 Reply Last reply
                      0
                      • Christian EhrlicherC Offline
                        Christian EhrlicherC Offline
                        Christian Ehrlicher
                        Lifetime Qt Champion
                        wrote on last edited by
                        #15

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

                        Qt Online Installer direct download: https://download.qt.io/official_releases/online_installers/
                        Visit the Qt Academy at https://academy.qt.io/catalog

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                        • Christian EhrlicherC 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 ...

                          D Offline
                          D Offline
                          davecotter
                          wrote on last edited by
                          #16

                          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

                          Kent-DorfmanK 1 Reply Last reply
                          0
                          • Christian EhrlicherC Offline
                            Christian EhrlicherC Offline
                            Christian Ehrlicher
                            Lifetime Qt Champion
                            wrote on last edited by
                            #17

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

                            Qt Online Installer direct download: https://download.qt.io/official_releases/online_installers/
                            Visit the Qt Academy at https://academy.qt.io/catalog

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                            • D 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

                              Kent-DorfmanK Offline
                              Kent-DorfmanK Offline
                              Kent-Dorfman
                              wrote on last edited by
                              #18

                              @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...

                              1 Reply Last reply
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                              • D davecotter

                                @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?

                                JonBJ Online
                                JonBJ Online
                                JonB
                                wrote on last edited by JonB
                                #19

                                @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.

                                kshegunovK 1 Reply Last reply
                                5
                                • D Offline
                                  D Offline
                                  davecotter
                                  wrote on last edited by
                                  #20

                                  see @JonB 's answer above! thanks!

                                  1 Reply Last reply
                                  2
                                  • JonBJ 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.

                                    kshegunovK Offline
                                    kshegunovK Offline
                                    kshegunov
                                    Moderators
                                    wrote on last edited by
                                    #21

                                    @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 ... ;)

                                    Read and abide by the Qt Code of Conduct

                                    JonBJ 1 Reply Last reply
                                    0
                                    • kshegunovK 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 ... ;)

                                      JonBJ Online
                                      JonBJ Online
                                      JonB
                                      wrote on last edited by JonB
                                      #22

                                      @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" :)

                                      kshegunovK 1 Reply Last reply
                                      0
                                      • JonBJ 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" :)

                                        kshegunovK Offline
                                        kshegunovK Offline
                                        kshegunov
                                        Moderators
                                        wrote on last edited by kshegunov
                                        #23

                                        @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.

                                        Read and abide by the Qt Code of Conduct

                                        JonBJ 1 Reply Last reply
                                        0
                                        • kshegunovK 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.

                                          JonBJ Online
                                          JonBJ Online
                                          JonB
                                          wrote on last edited by JonB
                                          #24

                                          @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
                                          

                                          ?

                                          kshegunovK 1 Reply Last reply
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