explain

问题 I have two tables in my database users and articles. Records in my users and articles table are given below: +----+--------+ | id | name | +----+--------+ | 1 | user1 | | 2 | user2 | | 3 | user3 | +----+--------+ +----+---------+----------+ | id | user_id | article | +----+---------+----------+ | 1 | 1 | article1 | | 2 | 1 | article2 | | 3 | 1 | article3 | | 4 | 2 | article4 | | 5 | 2 | article5 | | 6 | 3 | article6 | +----+---------+----------+ Given below the queries and the respected

问题 The queries are performed on a large table with 11 million rows. I have already performed an ANALYZE on the table prior to the query executions. Query 1: SELECT * FROM accounts t1 LEFT OUTER JOIN accounts t2 ON (t1.account_no = t2.account_no AND t1.effective_date < t2.effective_date) WHERE t2.account_no IS NULL; Explain Analyze: Hash Anti Join (cost=480795.57..1201111.40 rows=7369854 width=292) (actual time=29619.499..115662.111 rows=1977871 loops=1) Hash Cond: ((t1.account_no)::text = (t2

问题 I have a JsonB column in my table which holds hierarchical information. MyTable (id uuid, indexes jsonb, content bytea) Now if I create a CTE say WITH RECURSIVE hierarchy(pid, id, content) AS ( --load first parents SELECT t.indexes ->> 'parentId' as pId, t.id, t.content FROM MyTable c JOIN MyTable t ON t.indexes ->> 'Id' = c.indexes ->> 'parentId' WHERE c.Id = ANY('{..Some UUIDS}') UNION SELECT t.indexes ->> 'parentId' as pId, t.id, t.content FROM hierarchy h, MyTable t WHERE t.indexes ->>

问题 I have a strange performance problem with a oracle SQL statement. The statement is a more or less giantic subselect / inner join statement, therefore I'll only be able to post the structure of it here. It looks like this: SELECT "A".COL1, [...] FROM "A" INNER JOIN ( .. massive amount of subselects and joins ... ) WHERE [...] The statement is pretty fast for what it is doing (~30 Seconds). To further increase the speed I decided to restrict the selection by time: SELECT "A".COL1, [...] FROM "A

问题 I succesfully run: result = my_col.aggregate(my_pipeline, allowDiskUse=True) Now when I try: result = my_col.aggregate(my_pipeline, allowDiskUse=True, explain=True) it fails saying: pymongo.errors.ConfigurationError: The explain option is not supported. Use Database.command instead. Thus I try so as to add the needed option: result = mydb.command('aggregate', 'mycol', my_pipeline, {'explain':True}) but it fails saying: pymongo.errors.OperationFailure: 'pipeline' option must be specified as an

问题 I hava a table items and a table item_attributes . For simplicity let's say my table item has a column id and a column name . Of cource there is a index on the id column. the item_attributes table has the columns id , item_id , attribute_name and attribute_value and an index ON attrubute_name Now I want to query all items with a specific attribut without using a join. I do this with the following query: SELECT * FROM items i WHERE i.id IN ( SELECT item_id FROM item_attributes a WHERE a

问题 I've got a basic MySQL performance question related to explain. I have two queries that return the same result and I am trying to understand how to make sense of the EXPLAIN of the execution plans. The table has 50000 records in it and I am performing a record comparison. My first query takes 18.625 secs to run. The explain plan is as follows. id select_type table type possible_keys key key_len ref rows filtered Extra ---------------------------------------------------------------------------

问题 Consider the below Mongo index strategy and the query, Index: db.collec.ensureIndex({a:1,b:1,c:1}); Query: db.collec.find({"a":"valueA"},{"_id":0,"a":1,"c":1}).sort({"c":-1}).limit(150) The explain on the above query returns: /* 0 */ { "cursor" : "BtreeCursor a_1_b_1_c_1", "isMultiKey" : false, "n" : 150, "nscannedObjects" : 178, "nscanned" : 178, "nscannedObjectsAllPlans" : 279, "nscannedAllPlans" : 279, "scanAndOrder" : true, "indexOnly" : true, "nYields" : 0, "nChunkSkips" : 0, "millis" :

问题 According to the MySQL documentation regarding Optimizing Queries With Explain: * ALL: A full table scan is done for each combination of rows from the previous tables. This is normally not good if the table is the first table not marked const, and usually very bad in all other cases. Normally, you can avoid ALL by adding indexes that allow row retrieval from the table based on constant values or column values from earlier tables. Does this mean that any query that uses ALL can be optimized so

问题 I have a table that looks like so: ID | objectID | time | ... ------------------- 1 | 1 | ... 2 | 1 | ... 3 | 1 | ... 4 | 2 | ... 5 | 2 | ... 6 | 3 | ... 7 | 4 | ... ID is the primary key, objectID is non-unique. I am trying to increase performance on a query to get the most recent entries for all objectIDs, but the entries should not be newer than a certain value. I tried to following two queries, which should both provide the same (and correct results): SELECT * FROM ( SELECT * FROM table