A lot of what I do day-to-day is related to optimisation. Be it Perl code, SQL queries, Javascript or HTML there are usually at least a couple of cracking examples I find every week. On Friday I came across this:<\/p>\n
\r\nSELECT cycle FROM goldcrest WHERE id_run = ?<\/code>\r\n<\/pre>\nThis query is being used to find the number of the latest cycles (between 1 and 37 for each id_run) in a near-real-time tracking system and is used several times whenever a run report is viewed.<\/p>\n
\r\nEXPLAIN SELECT cycle FROM goldcrest WHERE id_run = 231;\r\n \r\n+----+-------------+-----------+------+---------------+---------+---------+-------+--------+-------------+\r\n| id | select_type | table \u00c2\u00a0 \u00c2\u00a0 | type | possible_keys | key \u00c2\u00a0 \u00c2\u00a0 | key_len | ref \u00c2\u00a0 | rows \u00c2\u00a0 | Extra \u00c2\u00a0 \u00c2\u00a0 \u00c2\u00a0 |\r\n+----+-------------+-----------+------+---------------+---------+---------+-------+--------+-------------+\r\n| \u00c2\u00a01 | SIMPLE \u00c2\u00a0 \u00c2\u00a0 \u00c2\u00a0| goldcrest | ref \u00c2\u00a0| g_idrun \u00c2\u00a0 \u00c2\u00a0 \u00c2\u00a0 | g_idrun | \u00c2\u00a0 \u00c2\u00a0 \u00c2\u00a0 8 | const | 262792 | Using where |\r\n+----+-------------+-----------+------+---------------+---------+---------+-------+--------+-------------+<\/code>\r\n<\/pre>\nIn itself this would be fine but the goldcrest table in this instance contains several thousand rows for each id_run. So, for id_run, let’s say, 231 this query happens to return approximately 588,000 rows to determine that the latest cycle for run 231 is the number 34.<\/p>\n
To clean this up we first try something like this:<\/p>\n
\r\nSELECT MIN(cycle),MAX(cycle) FROM goldcrest WHERE id_run = ?<\/code>\r\n<\/pre>\nwhich still scans the 588000 rows (keyed on id_run incidentally) but doesn’t actually return them to the user, only one row containing both values we’re interested in. Fair enough, the CPU and disk access penalties are similar but the data transfer penalty is significantly improved.<\/p>\n
Next I try adding an index against the id_run and cycle columns:<\/p>\n
\r\nALTER TABLE goldcrest ADD INDEX(id_run,cycle);\r\nQuery OK, 37589514 rows affected (23 min 6.17 sec)\r\nRecords: 37589514 \u00c2\u00a0Duplicates: 0 \u00c2\u00a0Warnings: 0<\/code>\r\n<\/pre>\nNow this of course takes a long time and, because the tuples are fairly redundant, creates a relatively inefficient index, also penalising future INSERTs. However, casually ignoring those facts, our query performance is now radically different:<\/p>\n
\r\nEXPLAIN SELECT MIN(cycle),MAX(cycle) FROM goldcrest WHERE id_run = 231;\r\n \r\n+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+\r\n| id | select_type | table | type | possible_keys | key \u00c2\u00a0| key_len | ref \u00c2\u00a0| rows | Extra \u00c2\u00a0 \u00c2\u00a0 \u00c2\u00a0 \u00c2\u00a0 \u00c2\u00a0 \u00c2\u00a0 \u00c2\u00a0 \u00c2\u00a0 \u00c2\u00a0 \u00c2\u00a0 \u00c2\u00a0 \u00c2\u00a0|\r\n+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+\r\n| \u00c2\u00a01 | SIMPLE \u00c2\u00a0 \u00c2\u00a0 \u00c2\u00a0| NULL \u00c2\u00a0| NULL | NULL \u00c2\u00a0 \u00c2\u00a0 \u00c2\u00a0 \u00c2\u00a0 \u00c2\u00a0| NULL | \u00c2\u00a0 \u00c2\u00a0NULL | NULL | NULL | Select tables optimized away |\r\n+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+\r\n \r\nSELECT MIN(cycle),MAX(cycle) FROM goldcrest WHERE id_run = 231;\r\n+------------+------------+\r\n| MIN(cycle) | MAX(cycle) |\r\n+------------+------------+\r\n| \u00c2\u00a0 \u00c2\u00a0 \u00c2\u00a0 \u00c2\u00a0 \u00c2\u00a01 | \u00c2\u00a0 \u00c2\u00a0 \u00c2\u00a0 \u00c2\u00a0 37 |\r\n+------------+------------+\r\n \r\n1 row in set (0.01 sec)<\/code>\r\n<\/pre>\nThat looks a lot better to me now!<\/p>\n
Generally I try to steer clear of the mysterious internal workings of database engines, but with much greater frequency come across examples like this:<\/p>\n
\r\nsub clone_type {\r\n my ($self, $clone_type, $clone) = @_;\r\n my %clone_type;\r\n\r\n if($clone and $clone_type) {\r\n $clone_type{$clone} = $clone_type;\r\n return $clone_type{$clone};\r\n }\r\n\r\n return;\r\n}<\/code>\r\n<\/pre>\nThankfully this one’s pretty quick to figure out – they’re usually *much* more convoluted, but still.. Huh??<\/p>\n
Pass in a clone_type scalar, create a local hash with the same name (Argh!), store the clone_type scalar in the hash keyed at position $clone, then return the same value we just stored.<\/p>\n
I don’t get it… maybe a global hash or something else would make sense, but this works out the same:<\/p>\n
\r\nsub clone_type {\r\n my ($self, $clone_type, $clone) = @_;\r\n\r\n if($clone and $clone_type) {\r\n return $clone_type;\r\n }\r\n return;\r\n}<\/code>\r\n<\/pre>\nand I’m still not sure why you’d want to do that if you have the values on the way in already.<\/p>\n
Programmers really need to think around the problem, not just through it. Thinking through may result in functionality but thinking around results in both function and performance which means a whole lot more in my book, and incidentally, why it seems so hard to hire good programmers.<\/p>\n","protected":false},"excerpt":{"rendered":"
A lot of what I do day-to-day is related to optimisation. Be it Perl code, SQL queries, Javascript or HTML there are usually at least a couple of cracking examples I find every week. On Friday I came across this: SELECT cycle FROM goldcrest WHERE id_run = ? This query is being used to find … Continue reading “Great pieces of code”<\/span><\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"om_disable_all_campaigns":false,"_monsterinsights_skip_tracking":false,"_uf_show_specific_survey":0,"_uf_disable_surveys":false,"footnotes":""},"categories":[11],"tags":[13,16,14,1081,15],"class_list":["post-13","post","type-post","status-publish","format-standard","hentry","category-programming","tag-code","tag-explain","tag-optimisation","tag-programming","tag-sql"],"aioseo_notices":[],"aioseo_head":"\n\t\t\n\t\n\t\n\t\n\t\n\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t