There is a certain unreality in comparing sets with null values in them, but if it happens in the heat of everyday database reporting, things can go very wrong. The two FULL OUTER JOIN queries were a bit slower, but it was a close race. The INTERSECT and INNER JOIN queries both used an inner merge join, and were close. ![]() We’ll check for variation by running the test several times. In fact, the first four all use the exact same ‘right anti semi merge join’ execution plan, and all take the same amount of time. However elegant your SQL, the optimizer merely shrugs and comes up with an efficient plan to execute it. How many words occur in Dracula that aren’t common words? Assuming there are no NULL values in the CommonWords.Word column (more on this later), then the following queries will return the same result (1555 words), and have the same execution plan, which uses a Merge Join (Right Anti Semi Join) between the two tables.Īll nine queries give the same results, but does any one approach perform better? Let’s put them all in a simple test harness to see how long each version takes! Again, the code download file includes the test harness code, and all nine queries.Īs the results show, although the queries look rather different, it’s generally just ‘syntactic sugar’ to the optimizer. It is useful, generally, to have tables like these in your sandbox server, for running tests while doing development work, though the book you use is your choice! The TestExistsAndIn download includes the script to create these two tables and populate each one from its associated text file. Imagine that we have two simple tables, one with all the common words in the English language ( CommonWords), and the other with a list of all the words in Bram Stoker’s ‘Dracula’ ( WordsInDracula). There are many ways of working out the differences between two data sets, but two of the most common are to use either the EXISTS or the IN logical operator. If so, you should consider using a NOT EXISTS operator instead of NOT IN, or recast the statement as a left outer join.Ī recommendation to prefer use of EXISTS over IN is included as a code analysis rule in SQL Prompt ( PE019). Nevertheless, you need to be cautious when using the NOT IN operator if the subquery’s source data contains NULL values. However, the query optimizer now treats EXISTS and IN the same way, whenever it can, so you’re unlikely to see any significant performance differences. For example, in cases where the query had to perform a certain task, but only if the subquery returned any rows, then when evaluating WHERE EXISTS (subquery), the database engine could quit searching as soon as it had found just one row, whereas WHERE IN (subquery) would always collect all the results from the sub-query, before further processing. ![]() It used to be that the EXISTS logical operator was faster than IN, when comparing data sets using a subquery. He is a regular contributor to Simple Talk and SQLServerCentral. Phil Factor (real name withheld to protect the guilty), aka Database Mole, has 30 years of experience with database-intensive applications.ĭespite having once been shouted at by a furious Bill Gates at an exhibition in the early 1980s, he has remained resolutely anonymous throughout his career. ( lz4 is available only if -with-lz4 was used when building PostgreSQL.This is a guest post from Phil Factor. The supported compression methods are pglz and lz4. (See ALTER TABLE for information on column storage modes.) Setting this property for a partitioned table has no direct effect, because such tables have no storage of their own, but the configured value will be inherited by newly-created partitions. Compression is supported only for variable-width data types, and is used only when the column's storage mode is main or extended. The COMPRESSION clause sets the compression method for the column. Use of EXTERNAL will make substring operations on very large text and bytea values run faster, at the penalty of increased storage space. ![]() EXTENDED is the default for most data types that support non- PLAIN storage. Writing DEFAULT sets the storage mode to the default mode for the column's data type. EXTERNAL is for external, uncompressed data, and EXTENDED is for external, compressed data. PLAIN must be used for fixed-length values such as integer and is inline, uncompressed. This controls whether this column is held inline or in a secondary TOAST table, and whether the data should be compressed or not. ![]() This form sets the storage mode for the column.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |