mysql - Addition of COUNT to query causes query to get stuck -


edit: simplifying it's trivial example:

select count(*) a_table use index(contract) left join b_table on a_table.contract = b_table.contract

also, nothing. explain:

enter image description here

i have 2 queries identical (b_table temporary table):

select if (b.contract null, 1, null) c a_table left join (select contract b_table) b on a.contract = b.contract

this query returns reliably in below .25 seconds. here explain:

enter image description here

here same query, count() around if:

select count(if (b.contract null, 1, null)) c a_table left join (select contract b_table) b on a.contract = b.contract

this 1 shows 'sending data' , @ least after several minutes, seems hang , nothing. here explain:

enter image description here

(note d == a_table a , eric_tmp == b_table. not confusing.)

this original query caused kerfuffle, above attempt rewrite it:

select count(*) a_table not exists (select 1 b_table b a.contract = b.contract)

which of course hung. heck going on here? i've tried adding , using non-unique index, doesn't seem help. don't understand why count cause problem here.

you consider first query done when returns first row, rather last row. second query needs go through data, before can return anything.

subqueries (which mysql calls "derived tables") affect performance. negatively.

your not exists query quite reasonable:

select count(*) a_table not exists (select 1 b_table b a.contract = b.contract);

for performance, want index on b_table(contract). fix performance problem, unless tables big.


Comments

Post a Comment

Popular posts from this blog

c++ - Difference between pre and post decrement in recursive function argument -

i have following sample code used pre-decrement void function(int c) { if(c == 0) { return; } else { cout << "dp" << c << endl; function(--c); cout << c << endl; return; } } this function give output input 4 : dp3 dp2 dp1 dp0 0 1 2 3 but when use post decrement void function(int c) { if(c == 0) { return; } else { cout << "dp" << c << endl; function(c--); cout << c << endl; return; } } output goes in infinite loop dp4 can explain me in detail why happens ? this happens because function(c--) called same value of c , when finishes c decremented. recursively called, hence called same value no chance of return , hit stack overflow error. assume this, int x = 1, y = 0; y = x++ result y == 1 , x == 2. if y = ++x , bot x , y 2.
Read more

php - Nothing but 'run(); ' when browsing to my local project, how do I fix this? -

i added new project. made mistake in projectname.conf file in sites-available apache2, led me reinstalling apache2. after reinstalling , re-enabling projects on every page of every project .. <body> run(); </body> .. i've run composer update, restarted service, restarted pc, checked config files, tried disabling new project. there nothing in error log, , access.log: 127.0.0.1 - - [15/may/2015:01:40:47 +0200] "get / http/1.1" 304 181 "-" "mozilla/5.0 (x11; ubuntu; linux x86_64; rv:38.0) gecko/20100101 firefox/38.0" maybe i'm forgetting apache2 reinstall? don't think did different last time installed it. looks haven't setup php correctly. take @ this great guide , maybe you're forgetting link php config apache2.
Read more

php - How can I echo out this array? -

i don't know how go grabbing data in variable , put foreach loop. class tree{ private $info = array( array('name'=>'jim','state'=>'nu','company'=>'nu','phone'=>array('cell'=>'5615111111','office'=>'5611111111'),'email'=>array('primary'=>'exs@example.com','ex@there.com')), array('name'=>'joe smith','city'=>'phoenix','phone'=>'4805551111','email'=>'jsmith@some_email.com'), array('name'=>'john doe','city'=>'chandler','company'=>'doe co.','email'=>array('jdoe@gmail.com','personal'=>'email@email.com'),'phone'=>'6025550002') ); } if you're inside of class, can access private variable using $this->variablename . example, i...
Read more