What is a cardinality estimator anyway?
What is a Cardinality Estimator anyway? The Cardinality Estimator (CE) predicts how many rows your query will likely return. The cardinality prediction is used by the query optimizer to generate an acceptable query plan. The more accurate the CE, the more optimal the query plan, at least that is the intent.
What is the difference between the CE and cardinality prediction?
The CE predicts how many rows your query will likely return. The cardinality prediction is used by the Query Optimizer to generate the optimal query plan. With more accurate estimations, the Query Optimizer can usually do a better job of producing a more optimal query plan.
Where is the cardinality estimator stored in SQL profiler?
The information regarding the cardinality estimator is stored in the explain plan for the query so let's add the "Showplan XML" event to a SQL Profiler session as shown in the screenshot below. You can select any extra columns you think would be helpful, but the data we're interested in is contained in the textdata column.
How does the optimizer determine cardinality?
The Optimizer determines the cardinality for each operation based on a complex set of formulas that use table and column level statistics as input (or the statistics derived by dynamic sampling). It’s considered the most important aspect of an execution plan because it strongly influences all of the other decisions the optimizer makes.
What is cardinality estimation in SQL Server?
Cardinality Estimation (CE) is how the Query Optimizer can estimate the total number of rows processed at each level of a query plan. Cardinality estimation in SQL Server is derived primarily from histograms created when indexes or statistics are created, either manually or automatically.
Why is cardinality estimation important?
If these initial steps produce inaccurate estimates all the next steps and the query execution may be very inefficient due to the inefficient plan. That is why cardinality estimation is so important mechanism in a query optimization process.
What is cardinality in query plan?
Cardinality is the estimated number of rows the step will return. Cost is the estimated amount of work the plan will do. A higher cardinality => you're going to fetch more rows => you're going to do more work => the query will take longer.
What is cardinality in Oracle database?
CARDINALITY returns the number of elements in a nested table. The return type is NUMBER .
What is the use of cardinality?
Database administrators may use cardinality to count tables and values. In a database, cardinality usually represents the relationship between the data in two different tables by highlighting how many times a specific entity occurs compared to another.
What are the different types of cardinality?
When dealing with columnar value sets, there are three types of cardinality: high-cardinality, normal-cardinality, and low-cardinality. High-cardinality refers to columns with values that are very uncommon or unique. High-cardinality column values are typically identification numbers, email addresses, or user names.
What is cardinality estimate Oracle?
What is a Cardinality Estimate? A cardinality estimate is the estimated number of rows, the optimizer believes will be returned by a specific operation in the execution plan.
What is cardinality and degree in SQL?
Answer: Cardinality refers to the number of tuples/rows in a table whereas, Degree refers to the number of attributes/columns in a table.
How do you reduce cardinality in a query?
The easiest and the quickest step you can take to reduce cardinality is to change your query parameter setting. You can reduce the number of possible values in the Page dimension by filtering out dynamic session/customer ID variables in the query parameter settings.
How do you calculate cardinality in SQL?
There is way to calculate cardinality in SQL statements which is : Select distinct Count(column_name) from Tablename; Definition of Database Cardinality for SQL Statements : The database cardinality is nothing but the uniqueness of values in SQL tables which helps to determine Query plan for performance optimization.
What is cardinality in data warehouse?
cardinality. From an OLTP perspective, this refers to the number of rows in a table. From a data warehousing perspective, this typically refers to the number of distinct values in a column. For most data warehouse DBAs, a more important issue is the degree of cardinality.
What is high cardinality data?
High cardinality describes databases with distinct values. For instance, if every line item had a unique ID number, description, email address, etc. A column with many repeated values would be described as having low cardinality.
How do you check cardinality in SQL?
There is way to calculate cardinality in SQL statements which is : Select distinct Count(column_name) from Tablename; Definition of Database Cardinality for SQL Statements : The database cardinality is nothing but the uniqueness of values in SQL tables which helps to determine Query plan for performance optimization.
What tool in the developer console contains information on SOQL query cardinality?
The Query Plan tool in the Developer Console can help speed up SOQL queries done over large volumes. Use the Query Plan tool to optimize and speed up queries done over large volumes.
How explain plan works in Oracle?
Running EXPLAIN PLAN EXPLAIN PLAN FOR SELECT last_name FROM employees; This explains the plan into the PLAN_TABLE table. You can then select the execution plan from PLAN_TABLE . This is useful if you do not have any other plans in PLAN_TABLE , or if you only want to look at the last statement.
What is a Cardinality Estimate?
A cardinality estimate is the estimated number of rows, the optimizer believes will be returned by a specific operation in the execution plan. The Optimizer determines the cardinality for each operation based on a complex set of formulas that use table and column level statistics as input (or the statistics derived by dynamic sampling).
What can cause a Cardinality Misestimate and how do I fix it?
Several factors can lead to incorrect cardinality estimates even when the basic table and column statistics are up to date. In part 5 of our series, I explain the leading causes of cardinality misestimates and how you can address them.
What is a Cardinality Estimator anyway?
The Cardinality Estimator (CE) predicts how many rows your query will likely return. The cardinality prediction is used by the query optimizer to generate an acceptable query plan. The more accurate the CE, the more optimal the query plan, at least that is the intent.
Is the new CE better than the legacy version?
If you are not having problems of the kind described here, leave the new CE alone. It is much better than the legacy version if it does not cause the problems we mentioned.
Is there a cardinality optimizer in SQL Server?
There has been a cardinality optimizer in SQL Server since Version 7 but we didn’t hear much about it until Microsoft made major changes to the CE in SQL 2014.
Targeting an individual query
Try levels 8, 10, and 11 to see if dynamic statistics help. If these tests do not improve the situation, it is unlikely that dynamic sampling will be a solution. Level 10 reads all blocks, which is OK in a test scenario, but it can take a long time and is not really intended to be a full-time setting.
An entire workload
Generally speaking, dynamic sampling is most appropriate for longer-running queries that access large volumes of data. It is less likely to be suitable for OLTP-style workloads where SQL statements should terminate in seconds (or sub-second), where we risk taking longer to parse a query than to execute it.
What to do if a database is legacy compatibility?
If all databases are in the current compatibility level, you’ll need to check for queries using trace flags to force SQL Server to use the legacy query optimizer.
Is the cardinality estimator still present in SQL Server 2014?
SQL Server 2014 introduced a brand new cardinality estimator. Unfortunately, the old cardinality estimator is still present. Queries will use the legacy cardinality estimator when the database is in an old compatibility level (less than 120 for SQL Server 2014) or a trace flag is being used.
Using Trace Flags
By using trace flags we can control the behavior of the optimizer for an individual query using the OPTION (QUERYTRACEON ####) hint.
Database Level Settings
There are two database level options for changing the optimizer's behavior. First is the compatibility level of the database which when updated will set certain behaviors in the database, in our case the optimizer, to the specified version.
Other query hints
Introduced in SQL Server 2016 was the USE HINT query argument. This hint is similar to the QUERYTRACEON hint as you specify it using the OPTION clause. "USE HINT ('FORCE_LEGACY_CARDINALITY_ESTIMATION')" corresponds to trace flag 9481 and "USE HINT ('FORCE_LEGACY_CARDINALITY_ESTIMATION')" corresponds to trace flag 2312.
What Version of the Cardinality Estimator is Being Used
In order to capture this information we will use one of my favorite tools, SQL Profiler. The information regarding the cardinality estimator is stored in the explain plan for the query so let's add the "Showplan XML" event to a SQL Profiler session as shown in the screenshot below.
