![]() It identifies the conditions of the where clause either as access or as filter predicates. The predicate information for the INDEX RANGE SCAN gives the crucial hint. SQL Server 2012 shows the seek predicates (=access predicates) using the row-value syntax. The following execution plan from the Oracle database unambiguously indicates that the EMP_TEST index starts with the DATE_OF_BIRTH column.Ģ | FETCH EMPLOYEES | 3 of 3 (100.00%) | 26ģ | IXSCAN EMP_TEST | 3 of 10000 (. With most databases you can even see this in the execution plan-you just have to know what to look for. To optimize performance, it is very important to know the scanned index range. The selectivity is of no use here, but one column order is still better than the other. This is the case regardless whether we filter by DATE_OF_BIRTH only or by SUBSIDIARY_ID only. If we look at the figures and consider the selectivity of the first column only, we see that both conditions match 13 records. With this example, we can also falsify the myth that the most selective column should be at the leftmost index position. The bigger the date range becomes, the bigger the performance difference will be. The difference can be negligible if the filter on DATE_OF_BIRTH is very selective on its own. The actual performance difference depends on the data and search criteria. Rule of thumb: index for equality first-then for ranges. Where will the database start to follow the leaf node chain, or to put it another way: where will the tree traversal end? For this illustration we search all employees of subsidiary 27 who were born between January 1 st and January 9 th 1971.įigure 2.2 visualizes a detail of the index on DATE_OF_BIRTH and SUBSIDIARY_ID-in that order. The following figures show the effect of the column order on the scanned index range. Of course an ideal index has to cover both columns, but the question is in which order? WHERE date_of_birth >= TO_DATE(?, 'YYYY-MM-DD')ĪND date_of_birth = TO_DATE(?, 'YYYY-MM-DD')ĪND date_of_birth <= TO_DATE(?, 'YYYY-MM-DD') The question is easy to answer if the SQL statement mentions the start and stop conditions explicitly: SELECT first_name, last_name, date_of_birth You can check that by asking yourself where an index scan starts and where it ends. It is therefore the golden rule of indexing to keep the scanned index range as small as possible. ![]() on SQL statements.The biggest performance risk of an INDEX RANGE SCAN is the leaf node traversal. ![]() In SQL, to compare one expression with another expression then we will be using mathematical operators like equal (=), greater than (>), less than (=), less than or equal to (), etc. Furthermore, the result of comparison in SQL can be TRUE, FALSE, or UNKNOWN (Note:- an operator that has one or two NULL expressions then it returns UNKNOWN). ![]() What is SQL Comparison OperatorsĬomparison operators in SQL are used when you want to check the conditions that compare one expression with another. Furthermore, it can be used on all expressions except expressions of the text, ntext, or image data types. SQL Comparison operators are used to test whether two expressions are the same. Summary: In this tutorial, you will learn the Types of SQL Comparison Operators (Transact-SQL) With Practical Example, and How to use SQL Comparison Operators to compare two values.
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