What does it mean to have a query cost?
Query cost is what optimizer thinks of how long your query will take (relative to total batch time). The optimizer tries to choose the optimal query plan by looking at your query and statistics of your data, trying several execution plans and selecting the least costly of them. Here you may read in more detail about how does it try to do this.
What should the total cost of each batch of SQL be?
Total cost of every batch, i.e. the sum of individual query costs should be 100%. However, it is possible that even in the actual query execution plans for costly queries, these costs are miscalculated and the sum is over 100% Children of the same parents are placed in the same column.
How is the relative cost of a query shown in Excel?
The arrows that represent a large number of rows are thicker If a query has multiple statements, query execution plans are shown in separate statements one under another. The relative cost for each query is presented as a percentage of total cost of all statements. The sum of all query costs is 100%
How is the cost of an on demand query calculated?
On-demand queries are charged based on the number of bytes read. For current on-demand query pricing, see the Pricing page. To estimate costs before running a query use the:
How is query cost relative to batch calculated?
As long as there is only one query in your batch, it’s cost relative to the batch will always be 100%. If you have more than one query in the batch, they will add up to 100%. The percentage only shows how queries in the batch relate to each other, it’s not an absolute measure of the cost.
How is query cost reported in execution plan?
The query cost is reported in execution plans as “estimated subtree cost”. This is an absolute figure such as 1.5. but now should be interpreted as a unitless measure of overall cost. The execution plan is a tree.
How are statistics used in a query optimizer?
The statistics improve the degree of accuracy of the measures. For the query shown in Example 4-1, the estimator uses selectivity, estimated cardinality (a total return of 10 rows), and cost measures to produce its total cost estimate of 3: The selectivity represents a fraction of rows from a row set.