Madyson  Reilly

Madyson Reilly

1596554400

Cluster Diagnostics: Troubleshoot Cluster Issues Using Only SQL Queries

TiDB is an open-source, distributed SQL database that supports Hybrid Transactional/Analytical Processing (HTAP) workloads. Ideally, a TiDB cluster should always be efficient and problem-free. It should be stable, load-balanced, and have a reliable rate of queries per second (QPS). There shouldn’t be any jitters (either in the cluster or on disk), and no hotspots, slow queries, or network fluctuations.

However, reality is often unsatisfactory. For external reasons, application traffic may surge and increase the pressure on the cluster. Through a chain reaction of events, the CPU load maxes out, out of memory errors occur, network latency increases, and disk writes and reads slow down.

Before TiDB 4.0, when these problems occurred in the cluster, there was no uniform way to locate them. We had to use various external tools to find problems in the cluster. It was tedious and time-consuming.

Now, TiDB 4.0 introduces a new feature, cluster diagnostics, a built-in widget in TiDB Dashboard, which lets you diagnose cluster problems within a specified time range and summarize the diagnostic results and cluster-related load monitoring information in a diagnostic report.

Our previous post gave you a quick peek at this feature. In this post, we’ll elaborate on cluster diagnostics’ diagnostic reports, and show you examples of how cluster diagnostics can help you quickly find system problems.

TiDB diagnosticsCluster diagnostic report

How Cluster Diagnostics Makes Your Job Easier

It’s important to provide a consistent user experience and reduce the learning curve. There is a lot of information that is relevant to troubleshooting, including cluster static information and cluster runtime information. We reorganize this information across the entire cluster to make sure that you can access it using SQL queries, without the need for external tools. At the same time, you can extract common SQL statements as scripts and write different troubleshooting scripts for different application scenarios.

Querying Logs With Cluster Diagnostics

In a TiDB cluster, a single transaction might involve multiple instances of TiDB’s storage engine, TiKV. Before TiDB 4.0, if you wanted to view logs related to a specific transaction ID ( txn_id), you might need to log in to all nodes and view the logs using the grep command. But in TiDB 4.0 and later, we offer cluster log tables. You can view all relevant logs with only one SQL statement. For example:

Plain Text

1

SELECT * FROM information_schema.cluster_log where message like "%{txn_id}%" and time > '2020-03-27 15:39:00' and time < '2020-03-27 15:50:00' 

Similarly, splitting and merging the basic unit of TiKV storage (the Region) and the Region leader switch usually involve multiple TiKV nodes. You can quickly view all activities in a life cycle of a Region through the log table.

When you query logs, the SQL statement’s predicates are pushed down to each log node for filtering, and no program collects all the logs. So the overhead is controllable and is lower than using the grep command. You can do what a distributed grep command can do but with less overhead.

Cluster Diagnostic Reports

In TiDB 4.0, if you want to diagnose or inspect the cluster within a time range, or check the load of the cluster, you can generate a diagnostic report for some time in TiDB Dashboard. The diagnostic report contains the diagnostic results during this period and monitoring and configuration information for each component in the system.

The Instance CPU Usage Report

The instance CPU usage report lets you view the average (AVG), maximum (MAX), and minimum (MIN) CPU usage for TiDB, Placement Driver (PD), and TiKV instances. You can use this report to quickly judge whether the cluster’s load is balanced or if it has hotspots.

CPU usage reportThe instance CPU usage report

The Monitoring Execution Time Report

The monitoring execution time report presents the monitoring time for each component in the cluster and what percentage it is of the total execution time for all queries. You can use this report to quickly determine whether a component’s execution time is too long and whether there is a bottleneck.

monitoring execution time report

#performance #sql #troubleshooting #database administration #distributed system #tidb

What is GEEK

Buddha Community

Cluster Diagnostics: Troubleshoot Cluster Issues Using Only SQL Queries
Cayla  Erdman

Cayla Erdman

1594369800

Introduction to Structured Query Language SQL pdf

SQL stands for Structured Query Language. SQL is a scripting language expected to store, control, and inquiry information put away in social databases. The main manifestation of SQL showed up in 1974, when a gathering in IBM built up the principal model of a social database. The primary business social database was discharged by Relational Software later turning out to be Oracle.

Models for SQL exist. In any case, the SQL that can be utilized on every last one of the major RDBMS today is in various flavors. This is because of two reasons:

1. The SQL order standard is genuinely intricate, and it isn’t handy to actualize the whole standard.

2. Every database seller needs an approach to separate its item from others.

Right now, contrasts are noted where fitting.

#programming books #beginning sql pdf #commands sql #download free sql full book pdf #introduction to sql pdf #introduction to sql ppt #introduction to sql #practical sql pdf #sql commands pdf with examples free download #sql commands #sql free bool download #sql guide #sql language #sql pdf #sql ppt #sql programming language #sql tutorial for beginners #sql tutorial pdf #sql #structured query language pdf #structured query language ppt #structured query language

Cayla  Erdman

Cayla Erdman

1596448980

The Easy Guide on How to Use Subqueries in SQL Server

Let’s say the chief credit and collections officer asks you to list down the names of people, their unpaid balances per month, and the current running balance and wants you to import this data array into Excel. The purpose is to analyze the data and come up with an offer making payments lighter to mitigate the effects of the COVID19 pandemic.

Do you opt to use a query and a nested subquery or a join? What decision will you make?

SQL Subqueries – What Are They?

Before we do a deep dive into syntax, performance impact, and caveats, why not define a subquery first?

In the simplest terms, a subquery is a query within a query. While a query that embodies a subquery is the outer query, we refer to a subquery as the inner query or inner select. And parentheses enclose a subquery similar to the structure below:

SELECT 
 col1
,col2
,(subquery) as col3
FROM table1
[JOIN table2 ON table1.col1 = table2.col2]
WHERE col1 <operator> (subquery)

We are going to look upon the following points in this post:

  • SQL subquery syntax depending on different subquery types and operators.
  • When and in what sort of statements one can use a subquery.
  • Performance implications vs. JOINs.
  • Common caveats when using SQL subqueries.

As is customary, we provide examples and illustrations to enhance understanding. But bear in mind that the main focus of this post is on subqueries in SQL Server.

Now, let’s get started.

Make SQL Subqueries That Are Self-Contained or Correlated

For one thing, subqueries are categorized based on their dependency on the outer query.

Let me describe what a self-contained subquery is.

Self-contained subqueries (or sometimes referred to as non-correlated or simple subqueries) are independent of the tables in the outer query. Let me illustrate this:

-- Get sales orders of customers from Southwest United States 
-- (TerritoryID = 4)

USE [AdventureWorks]
GO
SELECT CustomerID, SalesOrderID
FROM Sales.SalesOrderHeader
WHERE CustomerID IN (SELECT [CustomerID]
                     FROM [AdventureWorks].[Sales].[Customer]
                     WHERE TerritoryID = 4)

As demonstrated in the above code, the subquery (enclosed in parentheses below) has no references to any column in the outer query. Additionally, you can highlight the subquery in SQL Server Management Studio and execute it without getting any runtime errors.

Which, in turn, leads to easier debugging of self-contained subqueries.

The next thing to consider is correlated subqueries. Compared to its self-contained counterpart, this one has at least one column being referenced from the outer query. To clarify, I will provide an example:

USE [AdventureWorks]
GO
SELECT DISTINCT a.LastName, a.FirstName, b.BusinessEntityID
FROM Person.Person AS p
JOIN HumanResources.Employee AS e ON p.BusinessEntityID = e.BusinessEntityID
WHERE 1262000.00 IN
    (SELECT [SalesQuota]
    FROM Sales.SalesPersonQuotaHistory spq
    WHERE p.BusinessEntityID = spq.BusinessEntityID)

Were you attentive enough to notice the reference to BusinessEntityID from the Person table? Well done!

Once a column from the outer query is referenced in the subquery, it becomes a correlated subquery. One more point to consider: if you highlight a subquery and execute it, an error will occur.

And yes, you are absolutely right: this makes correlated subqueries pretty harder to debug.

To make debugging possible, follow these steps:

  • isolate the subquery.
  • replace the reference to the outer query with a constant value.

Isolating the subquery for debugging will make it look like this:

SELECT [SalesQuota]
    FROM Sales.SalesPersonQuotaHistory spq
    WHERE spq.BusinessEntityID = <constant value>

Now, let’s dig a little deeper into the output of subqueries.

Make SQL Subqueries With 3 Possible Returned Values

Well, first, let’s think of what returned values can we expect from SQL subqueries.

In fact, there are 3 possible outcomes:

  • A single value
  • Multiple values
  • Whole tables

Single Value

Let’s start with single-valued output. This type of subquery can appear anywhere in the outer query where an expression is expected, like the WHERE clause.

-- Output a single value which is the maximum or last TransactionID
USE [AdventureWorks]
GO
SELECT TransactionID, ProductID, TransactionDate, Quantity
FROM Production.TransactionHistory
WHERE TransactionID = (SELECT MAX(t.TransactionID) 
                       FROM Production.TransactionHistory t)

When you use a MAX() function, you retrieve a single value. That’s exactly what happened to our subquery above. Using the equal (=) operator tells SQL Server that you expect a single value. Another thing: if the subquery returns multiple values using the equals (=) operator, you get an error, similar to the one below:

Msg 512, Level 16, State 1, Line 20
Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression.

Multiple Values

Next, we examine the multi-valued output. This kind of subquery returns a list of values with a single column. Additionally, operators like IN and NOT IN will expect one or more values.

-- Output multiple values which is a list of customers with lastnames that --- start with 'I'

USE [AdventureWorks]
GO
SELECT [SalesOrderID], [OrderDate], [ShipDate], [CustomerID]
FROM Sales.SalesOrderHeader
WHERE [CustomerID] IN (SELECT c.[CustomerID] FROM Sales.Customer c
INNER JOIN Person.Person p ON c.PersonID = p.BusinessEntityID
WHERE p.lastname LIKE N'I%' AND p.PersonType='SC')

Whole Table Values

And last but not least, why not delve into whole table outputs.

-- Output a table of values based on sales orders
USE [AdventureWorks]
GO
SELECT [ShipYear],
COUNT(DISTINCT [CustomerID]) AS CustomerCount
FROM (SELECT YEAR([ShipDate]) AS [ShipYear], [CustomerID] 
      FROM Sales.SalesOrderHeader) AS Shipments
GROUP BY [ShipYear]
ORDER BY [ShipYear]

Have you noticed the FROM clause?

Instead of using a table, it used a subquery. This is called a derived table or a table subquery.

And now, let me present you some ground rules when using this sort of query:

  • All columns in the subquery should have unique names. Much like a physical table, a derived table should have unique column names.
  • ORDER BY is not allowed unless TOP is also specified. That’s because the derived table represents a relational table where rows have no defined order.

In this case, a derived table has the benefits of a physical table. That’s why in our example, we can use COUNT() in one of the columns of the derived table.

That’s about all regarding subquery outputs. But before we get any further, you may have noticed that the logic behind the example for multiple values and others as well can also be done using a JOIN.

-- Output multiple values which is a list of customers with lastnames that start with 'I'
USE [AdventureWorks]
GO
SELECT o.[SalesOrderID], o.[OrderDate], o.[ShipDate], o.[CustomerID]
FROM Sales.SalesOrderHeader o
INNER JOIN Sales.Customer c on o.CustomerID = c.CustomerID
INNER JOIN Person.Person p ON c.PersonID = p.BusinessEntityID
WHERE p.LastName LIKE N'I%' AND p.PersonType = 'SC'

In fact, the output will be the same. But which one performs better?

Before we get into that, let me tell you that I have dedicated a section to this hot topic. We’ll examine it with complete execution plans and have a look at illustrations.

So, bear with me for a moment. Let’s discuss another way to place your subqueries.

#sql server #sql query #sql server #sql subqueries #t-sql statements #sql

Introduction to Recursive CTE

This article will introduce the concept of SQL recursive. Recursive CTE is a really cool. We will see that it can often simplify our code, and avoid a cascade of SQL queries!

Why use a recursive CTE ?

The recursive queries are used to query hierarchical data. It avoids a cascade of SQL queries, you can only do one query to retrieve the hierarchical data.

What is recursive CTE ?

First, what is a CTE? A CTE (Common Table Expression) is a temporary named result set that you can reference within a SELECT, INSERT, UPDATE, or DELETE statement. For example, you can use CTE when, in a query, you will use the same subquery more than once.

A recursive CTE is one having a subquery that refers to its own name!

Recursive CTE is defined in the SQL standard.

How to make a recursive CTE?

A recursive CTE has this structure:

  • The WITH clause must begin with “WITH RECURSIVE”
  • The recursive CTE subquery has two parts, separated by “UNION [ALL]” or “UNION DISTINCT”:
  • The first part produces the initial row(s) for the CTE. This SELECT does not refer to the CTE name.
  • The second part recurses by referring to the CTE name in its FROM clause.

Practice / Example

In this example, we use hierarchical data. Each row can have zero or one parent. And it parent can also have a parent etc.

Create table test (id integer, parent_id integer);

insert into test (id, parent_id) values (1, null);

insert into test (id, parent_id) values (11, 1);
insert into test (id, parent_id) values (111, 11);

insert into test (id, parent_id) values (112, 11);

insert into test (id, parent_id) values (12, 1);

insert into test (id, parent_id) values (121, 12);

For example, the row with id 111 has as ancestors: 11 and 1.

Before knowing the recursive CTE, I was doing several queries to get all the ancestors of a row.

For example, to retrieve all the ancestors of the row with id 111.

While (has parent)

	Select id, parent_id from test where id = X

With recursive CTE, we can retrieve all ancestors of a row with only one SQL query :)

WITH RECURSIVE cte_test AS (
	SELECT id, parent_id FROM test WHERE id = 111
	UNION 
	SELECT test.id, test.parent_id FROM test JOIN cte_test ON cte_test.id = test.parent_id

) SELECT * FROM cte_test

Explanations:

  • “WITH RECURSIVE”:

It indicates we will make recursive

  • “SELECT id, parent_id FROM test WHERE id = 111”:

It is the initial query.

  • “UNION … JOIN cte_test” :

It is the recursive expression! We make a jointure with the current CTE!

Replay this example here

#sql #database #sql-server #sql-injection #writing-sql-queries #sql-beginner-tips #better-sql-querying-tips #sql-top-story

Cayla  Erdman

Cayla Erdman

1596441660

Welcome Back the T-SQL Debugger with SQL Complete – SQL Debugger

When you develop large chunks of T-SQL code with the help of the SQL Server Management Studio tool, it is essential to test the “Live” behavior of your code by making sure that each small piece of code works fine and being able to allocate any error message that may cause a failure within that code.

The easiest way to perform that would be to use the T-SQL debugger feature, which used to be built-in over the SQL Server Management Studio tool. But since the T-SQL debugger feature was removed completely from SQL Server Management Studio 18 and later editions, we need a replacement for that feature. This is because we cannot keep using the old versions of SSMS just to support the T-SQL Debugger feature without “enjoying” the new features and bug fixes that are released in the new SSMS versions.

If you plan to wait for SSMS to bring back the T-SQL Debugger feature, vote in the Put Debugger back into SSMS 18 to ask Microsoft to reintroduce it.

As for me, I searched for an alternative tool for a T-SQL Debugger SSMS built-in feature and found that Devart company rolled out a new T-SQL Debugger feature to version 6.4 of SQL – Complete tool. SQL Complete is an add-in for Visual Studio and SSMS that offers scripts autocompletion capabilities, which help develop and debug your SQL database project.

The SQL Debugger feature of SQL Complete allows you to check the execution of your scripts, procedures, functions, and triggers step by step by adding breakpoints to the lines where you plan to start, suspend, evaluate, step through, and then to continue the execution of your script.

You can download SQL Complete from the dbForge Download page and install it on your machine using a straight-forward installation wizard. The wizard will ask you to specify the installation path for the SQL Complete tool and the versions of SSMS and Visual Studio that you plan to install the SQL Complete on, as an add-in, from the versions that are installed on your machine, as shown below:

Once SQL Complete is fully installed on your machine, the dbForge SQL Complete installation wizard will notify you of whether the installation was completed successfully or the wizard faced any specific issue that you can troubleshoot and fix easily. If there are no issues, the wizard will provide you with an option to open the SSMS tool and start using the SQL Complete tool, as displayed below:

When you open SSMS, you will see a new “Debug” tools menu, under which you can navigate the SQL Debugger feature options. Besides, you will see a list of icons that will be used to control the debug mode of the T-SQL query at the leftmost side of the SSMS tool. If you cannot see the list, you can go to View -> Toolbars -> Debugger to make these icons visible.

During the debugging session, the SQL Debugger icons will be as follows:

The functionality of these icons within the SQL Debugger can be summarized as:

  • Adding Breakpoints to control the execution pause of the T-SQL script at a specific statement allows you to check the debugging information of the T-SQL statements such as the values for the parameters and the variables.
  • Step Into is “navigate” through the script statements one by one, allowing you to check how each statement behaves.
  • Step Over is “execute” a specific stored procedure if you are sure that it contains no error.
  • Step Out is “return” from the stored procedure, function, or trigger to the main debugging window.
  • Continue executing the script until reaching the next breakpoint.
  • Stop Debugging is “terminate” the debugging session.
  • Restart “stop and start” the current debugging session.

#sql server #sql #sql debugger #sql server #sql server stored procedure #ssms #t-sql queries

Madyson  Reilly

Madyson Reilly

1596554400

Cluster Diagnostics: Troubleshoot Cluster Issues Using Only SQL Queries

TiDB is an open-source, distributed SQL database that supports Hybrid Transactional/Analytical Processing (HTAP) workloads. Ideally, a TiDB cluster should always be efficient and problem-free. It should be stable, load-balanced, and have a reliable rate of queries per second (QPS). There shouldn’t be any jitters (either in the cluster or on disk), and no hotspots, slow queries, or network fluctuations.

However, reality is often unsatisfactory. For external reasons, application traffic may surge and increase the pressure on the cluster. Through a chain reaction of events, the CPU load maxes out, out of memory errors occur, network latency increases, and disk writes and reads slow down.

Before TiDB 4.0, when these problems occurred in the cluster, there was no uniform way to locate them. We had to use various external tools to find problems in the cluster. It was tedious and time-consuming.

Now, TiDB 4.0 introduces a new feature, cluster diagnostics, a built-in widget in TiDB Dashboard, which lets you diagnose cluster problems within a specified time range and summarize the diagnostic results and cluster-related load monitoring information in a diagnostic report.

Our previous post gave you a quick peek at this feature. In this post, we’ll elaborate on cluster diagnostics’ diagnostic reports, and show you examples of how cluster diagnostics can help you quickly find system problems.

TiDB diagnosticsCluster diagnostic report

How Cluster Diagnostics Makes Your Job Easier

It’s important to provide a consistent user experience and reduce the learning curve. There is a lot of information that is relevant to troubleshooting, including cluster static information and cluster runtime information. We reorganize this information across the entire cluster to make sure that you can access it using SQL queries, without the need for external tools. At the same time, you can extract common SQL statements as scripts and write different troubleshooting scripts for different application scenarios.

Querying Logs With Cluster Diagnostics

In a TiDB cluster, a single transaction might involve multiple instances of TiDB’s storage engine, TiKV. Before TiDB 4.0, if you wanted to view logs related to a specific transaction ID ( txn_id), you might need to log in to all nodes and view the logs using the grep command. But in TiDB 4.0 and later, we offer cluster log tables. You can view all relevant logs with only one SQL statement. For example:

Plain Text

1

SELECT * FROM information_schema.cluster_log where message like "%{txn_id}%" and time > '2020-03-27 15:39:00' and time < '2020-03-27 15:50:00' 

Similarly, splitting and merging the basic unit of TiKV storage (the Region) and the Region leader switch usually involve multiple TiKV nodes. You can quickly view all activities in a life cycle of a Region through the log table.

When you query logs, the SQL statement’s predicates are pushed down to each log node for filtering, and no program collects all the logs. So the overhead is controllable and is lower than using the grep command. You can do what a distributed grep command can do but with less overhead.

Cluster Diagnostic Reports

In TiDB 4.0, if you want to diagnose or inspect the cluster within a time range, or check the load of the cluster, you can generate a diagnostic report for some time in TiDB Dashboard. The diagnostic report contains the diagnostic results during this period and monitoring and configuration information for each component in the system.

The Instance CPU Usage Report

The instance CPU usage report lets you view the average (AVG), maximum (MAX), and minimum (MIN) CPU usage for TiDB, Placement Driver (PD), and TiKV instances. You can use this report to quickly judge whether the cluster’s load is balanced or if it has hotspots.

CPU usage reportThe instance CPU usage report

The Monitoring Execution Time Report

The monitoring execution time report presents the monitoring time for each component in the cluster and what percentage it is of the total execution time for all queries. You can use this report to quickly determine whether a component’s execution time is too long and whether there is a bottleneck.

monitoring execution time report

#performance #sql #troubleshooting #database administration #distributed system #tidb