query-optimization

问题 I've been working with databases for a long time but I'm new to query optimization. I have the following query (some of it code-generated): SELECT DISTINCT COALESCE(gi.start_time, '') start_time, COALESCE(b.name, '') bank, COALESCE(a.id, '') account_id, COALESCE(a.account_number, '') account_number, COALESCE(at.code, '') account_type, COALESCE(a.open_date, '') open_date, COALESCE(a.interest_rate, '') interest_rate, COALESCE(a.maturity_date, '') maturity_date, COALESCE(a.opening_balance, '')

问题 When I execute this query like this they take so much execution time because user_fans table contain 10000 users entries. How can I optimize it? Query SELECT uf.`user_name`,uf.`user_id`, @post := (SELECT COUNT(*) FROM post WHERE user_id = uf.`user_id`) AS post, @post_comment_likes := (SELECT COUNT(*) FROM post_comment_likes WHERE user_id = uf.`user_id`) AS post_comment_likes, @post_comments := (SELECT COUNT(*) FROM post_comments WHERE user_id = uf.`user_id`) AS post_comments, @post_likes :=

问题 My question is regarding the handling of MySQL index on VARCHAR combined with an int COLUMN when using prefix matching. e.g. if I have such a query: SELECT * FROM tbl WHERE name LIKE 'query%' ORDER BY weight DESC LIMIT 5 Considering I have one index one name->weight, does that index need to find all apperances of the prefix query and then ORDER BY, or does he keeps the cross calculation indexed even with the use of prefix matching (%). I'm troubled by it, because for popular names (e.g. query

问题 I'm trying to optimize up some horrendously complicated SQL queries because it takes too long to finish. In my queries, I have dynamically created SQL statements with lots of the same functions, so I created a temporary table where each function is only called once instead of many, many times - this cut my execution time by 3/4. So my question is, can I expect to see much of a difference if say, 1,000 datediff computations are narrowed to 100? EDIT: The query looks like this : SELECT DISTINCT

问题 I need to look up an ID in a very large database. The ID is: 0167a901-e343-4745-963c-404809b74dd9 The database has hundreds of tables, and millions of rows in the big tables. I can narrow the date to within the last 2 or 3 months, but that's about it. I'm looking for any clues as to how to narrow down searches like this. One thing I'm curious about is whether using LIKE searches helps. i.e does it help to do something like select top 10 * from BIG_TABLE where DESIRED_ID like '016%' Any tips

问题 I'm doing some refactoring on a Oracle Schema (oracle version 10), and I see a lot of views that use hints *+ALL_ROWS*/ . In others views there are also other kind of hints. Why I should use an hints? the DB doesn't make the best choice in base of the query? many thanks! 回答1: That's a good question, but there's no single answer to it because there are different categories of hint for which different advice would apply. http://docs.oracle.com/cd/E11882_01/server.112/e16638/hintsref.htm

问题 Query SELECT id FROM `user_tmp` WHERE `code` = '9s5xs1sy' AND `go` NOT REGEXP 'http://www.xxxx.example.com/aflam/|http://xx.example.com|http://www.xxxxx..example.com/aflam/|http://www.xxxxxx.example.com/v/|http://www.xxxxxx.example.com/vb/' AND check='done' AND `dataip` <1319992460 ORDER BY id DESC LIMIT 50 MySQL returns: Showing rows 0 - 29 ( 50 total, Query took 21.3102 sec) [id: 2622270 - 2602288] Query took 21.3102 sec if i remove AND dataip <1319992460 MySQL returns Showing rows 0 - 29 (

问题 What is 'correct' query to fetch a cumulative sum in MySQL? I've a table where I keep information about files, one column list contains the size of the files in bytes. (the actual files are kept on disk somewhere) I would like to get the cumulative file size like this: +------------+---------+--------+----------------+ | fileInfoId | groupId | size | cumulativeSize | +------------+---------+--------+----------------+ | 1 | 1 | 522120 | 522120 | | 2 | 2 | 316042 | 316042 | | 4 | 2 | 711084 |

问题 Consider the following 2 tables: Table A: id event_time Table B id start_time end_time Every record in table A is mapped to exactly 1 record in table B. This means table B has no overlapping periods. Many records from table A can be mapped to the same record in table B. I need a query that returns all A.id, B.id pairs. Something like: SELECT A.id, B.id FROM A, B WHERE A.event_time BETWEEN B.start_time AND B.end_time I am using MySQL and I cannot optimize this query. With ~980 records in table

问题 Consider the following 2 tables: Table A: id event_time Table B id start_time end_time Every record in table A is mapped to exactly 1 record in table B. This means table B has no overlapping periods. Many records from table A can be mapped to the same record in table B. I need a query that returns all A.id, B.id pairs. Something like: SELECT A.id, B.id FROM A, B WHERE A.event_time BETWEEN B.start_time AND B.end_time I am using MySQL and I cannot optimize this query. With ~980 records in table