mysql - Select from table is slow -


my problem is: simple select query takes long time (3 minutes).

structure:

mysql> show create table seventhcont_exceptionreport;  seventhcont_exceptionreport | create table `seventhcont_exceptionreport` (   `id` int(11) not null auto_increment,   `body_html` longtext not null,   `datetime_created` datetime not null,   `subject` varchar(256) not null,   `host` varchar(128) not null,   `exc_value` varchar(512) default null,   primary key (`id`) ) engine=myisam auto_increment=74607 default charset=utf8 |

rows count:

mysql> select count(*) seventhcont_exceptionreport;  +----------+ | count(*) | +----------+ |     7064 | +----------+ 1 row in set (0.00 sec)

query 1 (normal):

mysql> select id, datetime_created seventhcont_exceptionreport order id limit 100 offset 6000;  ... 100 rows in set (0.30 sec)

query 2 (very slow):

mysql> select id, datetime_created seventhcont_exceptionreport order id limit 100 offset 7000;  ...  63 rows in set (3 min 40.56 sec)

!!! 3 minutes , 40 sec.

why?

update

explain query 1:

mysql> explain select id, datetime_created seventhcont_exceptionreport order id limit 100 offset 6000; +----+-------------+-----------------------------+-------+---------------+---------+---------+------+------+-------+ | id | select_type | table                       | type  | possible_keys | key     | key_len | ref  | rows | | +----+-------------+-----------------------------+-------+---------------+---------+---------+------+------+-------+ |  1 | simple      | seventhcont_exceptionreport | index | null          | primary | 4       | null | 6100 |       | +----+-------------+-----------------------------+-------+---------------+---------+---------+------+------+-------+ 1 row in set (0.00 sec)

explain query 2:

mysql> explain select id, datetime_created seventhcont_exceptionreport order id limit 100 offset 7000; +----+-------------+-----------------------------+------+---------------+------+---------+------+------+----------------+ | id | select_type | table                       | type | possible_keys | key  | key_len | ref  | rows |          | +----+-------------+-----------------------------+------+---------------+------+---------+------+------+----------------+ |  1 | simple      | seventhcont_exceptionreport |  | null          | null | null    | null | 7067 | using filesort | +----+-------------+-----------------------------+------+---------------+------+---------+------+------+----------------+ 1 row in set (0.00 sec)

update

analyze table:

mysql> analyze table seventhcont_exceptionreport; +--------------------------------+---------+----------+----------+ | table                          | op      | msg_type | msg_text | +--------------------------------+---------+----------+----------+ | 7k.seventhcont_exceptionreport | analyze | status   | ok       | +--------------------------------+---------+----------+----------+ 1 row in set (2.51 sec)

i no mysql specialist, might able point in right direction.

in first query, can see in explain plan, index access used. in contrast, second query, can see non-index access performed (type index vs all). also, can see mysql using using filesort.

this means mysql cannot perform sort operation on index , therefore performing on data itself. because sort buffer small (also see https://www.percona.com/blog/2009/03/05/what-does-using-filesort-mean-in-mysql/).

therefore, try increase size of sort buffer (soft_buffer_size).


Comments

Post a Comment

Popular posts from this blog

authentication - Mongodb revoke acccess to connect test database -

i have ec2 instance mongodb 3.2.0 installed. have user admin access. can connect user , perform operation remotely. 1 can connect database test remote if knows ip mongo 11.11.11.11. although not able make operations, want restrict access user credentials can connect server. first need create users , grant roles users, example: use admin db.createuser( { user: "myuseradmin", pwd: "abc123", roles: [ { role: "useradminanydatabase", db: "admin" } ] } ) then need start server --auth parameter mongod --auth more information here
Read more

r - Update two sets of radiobuttons reactively - shiny -

i have read ( how make choices in radiobuttons reactive in shiny? ) shows me how update radiobutton s in reactive way. however, when try , update 2 sets of buttons same data, 1 set renders. example: server: # create example data wafer <- rep(c(1:3), each=3) length <- c(1,1,2,1,1,1,3,5,1) width <- c(3,1,6,1,1,1,1,1,6) dd <- data.frame(wafer, length, width) shinyserver(function(input, output, session){ # create reactive dataframe store data values <- reactivevalues() values$df <- data.frame() # lengths , widths of wafer user input <- eventreactive(input$do, { subset(dd, wafer %in% input$wafer, select = length:width) }) # update reactive data frame lengths , widths have been selected user input observe({ if(!is.null(a())) { values$df <- rbind(isolate(values$df), a()) } }) output$wl <- renderdatatable({ a() }) # update radio buttons unique length , widths stored in values$df # ever "observe" put first in co...
Read more

ios - Realm over CoreData should I use NSFetchedResultController or a Dictionary? -

i'm working on app using core data , nsfetchedresultcontroller many reasons switch use realm. saw there "realm version" of nsfetchedresultcontroller on github wouldn't compatible current code using core data. the view controller displaying list of people same school, address book. list sublist of people studied in same city. so thinking make 1 unique request database retrieve list of people , filter locally list within 1 dictionary per school [string: anyobject] string section name within tableview , anyobject array of people. first solution, make nsfetchedresultcontroller each school predicate filtering location. delete actions etc.. handled delegates -> not compatible realm create dictionaries , update them each actions... -> works realm it's annoying code. any better solution? edit: i need clarify request: i'd write class inherit uitableviewcontroller. this class has list of people sorted in alphabetical order the tableview has ...
Read more