The index first and foremost puts the rows in DateAppointed order, then after that, it sorts by DateLeft, so if you want to scan all dateleft values (which your second where clause would require), the data is not ordered by DateLeft in your index, so you need to do multiple scans.īecause you already reduced the scope of your data with your first where clause, in reality the query is probably quite quick, but on a very very large dataset this would still take hours. So the first WHERE clause would be dealt with efficiently, the optimiser can make best use of your index to find all rows which match the WHERE DateAppointed'', you are at the mercy of full table scans again, albeit this time you're doing full table scans on your index. The way you have it in your example you made an index on both dateleft and dateappointed, meaning the order of dateleft will be completely dependant on dateappointed You should have actually created two indexes, one for DateLeft and one for DateAppointed. 6 Answers Sorted by: 74 According to the manual: create table and create table as you can either: create table with primary key first, and use select into later create table as first, and use add primary key later But not both create table as with primary key - what you wanted. The solution was to specify an index which had include columns covering the columns specified in the It was a third party supplied database and it wasn’t practical to make structural changes. If this was a database we owned then we could have added a primary key, either by identifying anĮxisting column to use or to add an extra column and specifying it as an identity column. Specified in the select statement, but it required a primary key on the table to be able to do that look up. Table to meet the where condition, it would then have to go back to the table to get the columns The PRIMARY KEY constraint specifies that a column or columns of a table can contain only unique (non-duplicate), nonnull values. The problem was that although the optimiser could use the index to identify the correct rows in the Problem was – there was no primary key on the table. At this point I suddenly realised what the Trying a few things and not succeeding in getting the query to use the index I decided to see what theĭatabase Engine Tuning Advisor (DETA) made of the query. A table cannot have more than 1600 columns. This makes the combination of inheritance and unique constraints rather dysfunctional. (See CREATE INDEX for more information.) Unique constraints and primary keys are not inherited in the current implementation. On the face of there was no obvious reason why the query should not have made use of this index. Thus, it is not necessary to create an index explicitly for primary key columns. In the ‘Execution Plan’ tab in SSMS confirmed that the query is table scanning : Switching on the ‘Include Actual Execution Plan’, running the query and looking at the graphical output Indexes that are not created for a constraint can be reindexed in the same way.CREATE NONCLUSTERED INDEX idx1 ON dbo. PostgreSQL automatically creates a unique index when a unique constraint or primary key is defined for a table. Recreating Indexes with Foreign Key constraints You can also create temporary tables that will. These definitions can be used as is when we want to recreate them. create table users ( id serial primary key, name character varying not null, active boolean default true ).
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |