indexing

问题 In MySQL " An index key for a single-column index can be up to 767 bytes. The same length limit applies to any index key prefix. " Reviewing and searching the MariaDB documentation I cannot find a reference to this, or a similar, limit. What is this limit? What is the link to the documentation page I didn't find? 回答1: It is a bit hidden, but e.g. https://mariadb.com/kb/en/mariadb/documentation/storage-engines/converting-tables-from-myisam-to-innodb/ says: The maximum length of an INDEX is

问题 I noticed that if I enter the value 'seasons' in a full text search enabled string field of some collection, then MongoDB finds this value when I query for 'season'. But if I enter something more complex like e.g. 'mice' or 'criteria', it does not find these values when I query for 'mouse' or 'criterion' respectively. Is that normal and are there any clear rules what MongoDB is able to stem and what not? [test] 2014-03-30 18:25:09.551 >>> db.TestFullText7.find(); { "_id" : ObjectId(

问题 Part of me is worries that this question will get closed, but I'm genuinely baffled by something. In every language's regex that I've used, the capturing groups are indexed at one, even when the rest of the language is indexed at zero. I thought of design decisions that would lead to 1-indexing, which is usually to lower the barrier to entry for non-technical people, however when it comes to regex, which is already hellish and incomprehensible, this argument doesn't really seem to hold.

问题 If there is an index on page_type, our_id, date and when querying, db.analytics.find({page_type: 'ingredients', ga_date: {$gte : new Date('Wed Sep 08 2010 12:00:00 GMT-0800')}}) db.analytics.find({page_type: 'ingredients', ga_date: {$gte : new Date('Wed Sep 08 2010 12:00:00 GMT-0800')}}).explain() if our_id is omitted, or date is omitted, it can still use the index, with something like the following in the explain() output: "our_id" : [ [ { "$minElement" : 1 }, { "$maxElement" : 1 } ] ], even

问题 Why does INDEX creation statement have UNIQUE argument? As I understand, the non-clustered index contains a bookmark, a pointer to a row, which should be unique to distinguish even non-unique rows, so insuring non-clustered index to be unique ? Correct? So, do I understand that no-unique index can be only on clustered table? since "A clustered index on a view must be unique" [1] Since "The bottom, or leaf, level of the clustered index contains the actual data rows of the table" [1], do I

问题 I've got a MongoDB collection that holds about 100M documents. The documents basically look like this: _id : ObjectId("asd1234567890") _reference_1_id : ObjectId("fgh4567890123") _reference_2_id : ObjectId("jkl7890123456") name : "Test1" id : "4815162342" created_time : Date( 1331882436000 ) _contexts : ["context1", "context2"] ... There are some indexes set, here's the output of db.mycoll.getIndexes(); [ { "v" : 1, "key" : { "_id" : 1 }, "ns" : "mydb.mycoll", "name" : "_id_" }, { "v" : 1,

问题 Here is the table definition CREATE TABLE `dt_prdtime` ( `TCompany` varchar(3) NOT NULL DEFAULT '', `TPerCode` varchar(8) NOT NULL, `TBegDateTime` datetime NOT NULL DEFAULT '0000-00-00 00:00:00' COMMENT 'วันที่', `TQPay` int(1) NOT NULL DEFAULT '2', `TYear` int(4) NOT NULL, `TMonth` int(2) NOT NULL, PRIMARY KEY (`TCompany`,`TPerCode`,`TBegDateTime`), KEY `TMonth` (`TMonth`) USING BTREE, KEY `TPerCode` (`TPerCode`,`TYear`,`TMonth`) ) ENGINE=MyISAM DEFAULT CHARSET=utf8 And this is data sample.

问题 I'm deploying a Rails application that aggregates coupon data from various third-party providers into a searchable database. Searches are conducted across four fields for each coupon: headline, coupon code, description, and expiration date. Because some of these third-party providers do a rather bad job of keeping their data sorted, and because I don't want duplicate coupons to creep into my database, I've implemented a unique compound index across those four columns. That prevents the same

问题 I'm deploying a Rails application that aggregates coupon data from various third-party providers into a searchable database. Searches are conducted across four fields for each coupon: headline, coupon code, description, and expiration date. Because some of these third-party providers do a rather bad job of keeping their data sorted, and because I don't want duplicate coupons to creep into my database, I've implemented a unique compound index across those four columns. That prevents the same

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