sql - Performance selecting rows not matching entry in another table when NULL is present -

-

related question: how select rows no matching entry in table?

i trying select rows using method , couldn't work in sqlite. after bit of wrangling occurred me reason might there null values in fields. sure enough, right, , when changed = is in query below things started behaving expected:

create temp table newevent(id integer,t integer,name,extra,extra2,extra3); insert newevent(id,t,name,extra,extra2,extra3) values                            (0, 1376351146, 'test', null, null, null),                            (0, 1376348867, 'old', null, null,null); select n.id,n.t,n.name,n.extra,n.extra2,n.extra3 newevent n         left join event e on n.t = e.t , n.name e.name                                       , n.extra e.extra;                                       , n.extra2 e.extra2;                                       , n.extra3 e.extra3        e.id null; drop table newevent;

in above example, there existing record in table event name='old'. newevent table defined same original event table.

however, noticed big problem: query taking 30 seconds run! if change only n.name e.name n.name = e.name leave other ises as-is, query takes around 400ms. (there around 3 million records in table event.)

why big difference in performance? turns out can use = instead of is name comparison because never null, if ever null seems break. conversely, concerned @ point query might start running slow, since don't understand name makes equality query run faster. guess maybe sqlite somehow knows there nulls in fields , able optimize bit more firm wild guess.

as far can tell, is = additional provisio treat null comparisions same if empty strings (assuming there no actual empty strings compare). why using = on name field 75 times faster, has no effect on performance on fields???

in join, sqlite can optimize = index lookups, not is. furthermore, not possible use more 1 index per table in single query.

so either not have multi-column index includes both name , extra*, or selectivity of additional column(s) not high enough matter.

you try different query using compound select:

select t, name, extra, extra2, extra3 newevent except select t, name, extra, extra2, extra3 event

however, not allow column not compard (like id).


Comments

Post a Comment

Popular posts from this blog

Issues changing the value of an element in an array in matlab -

-
i'm trying create program sum n-1 , n-2 numbers create n , store value in array. currently, output (assuming numdisp > 2 ) num1 , num2 , 0 , 0 , 0 ... size of array correct, i'm having issues reassigning elements. num1 = input('input first integer of fibonacci sequence: '); num2 = input('input second integer of fibonacci sequence: '); numsdisp = input('how many integers display? --> '); list = zeros(numsdisp,1); list(1) = num1; list(2) = num2; if (numsdisp <=0) disp('value must integer greater zero.') elseif (numsdisp == 1) disp(num1) elseif (numsdisp == 2) disp(num1) disp(num2) else disp(num1) disp(num2) index = 3:numsdisp disp(list(index)) list(index) = list((index-1)) + list((index-2)); end end example input: 3 3 4 corresponding output: 3 3 0 0 you aren't printing out final result, intermediate steps. in for loop, printing list(index) updating value, rather updat...
Read more

php - MySQLi binding parameters in a prepared statement doesn't work unless inserted after "WHERE" -

-
i have php code pulls info database. uses prepared statement, works fine written below: <?php $n = '5'; if ($stmt = $connection->prepare("select title items id = ?")) { $stmt->bind_param("s", $n); $stmt->execute(); $stmt->bind_result($title); while ($stmt->fetch()) { echo $title; } $stmt->close(); } ?> and let's echo "batman". however, when move placeholder here: "select ? items id = 5" and change: $n = 'title'; instead of echoing "batman", echoes "title". not possible use placeholder select parameter? what have done this: select "title" items id = 5 you cant use prepared statement bindings identifiers, parameters.
Read more

vb.net - Alternative to the T-SQL AS keyword -

-
please see code below: imports system.data.sqlclient imports system.configuration public class form1 private _constring string private sub form1_load(byval sender object, byval e system.eventargs) handles me.load dim objdr sqldatareader dim objcommand sqlcommand dim objcon sqlconnection dim id integer try _constring = configurationmanager.connectionstrings("testconnection").tostring objcon = new sqlconnection(_constring) objcommand = new sqlcommand("select person.urn, car.urn person inner join car on person.urn = car.urn , personid=1") objcommand.connection = objcon objcon.open() objdr = objcommand.executereader(connectionstate.closed) while objdr.read id = objdr("urn") 'line 19 loop objdr.close() ...
Read more