How does MySQL implement sorting of data?

1. Background

Maybe you were asked about mysql sorting in your interviews

Or perhaps you thought about how database sorting is implemented when you were learning about sorting algorithms?

The following focuses on answering this question from an interview perspective

2. Interview answers

1. General interviewer responses

The average interviewer's response is usually a point-to-point answer, as follows.

MySQL implements sorting of data through sorting algorithms and index structures. Sorting is a very common operation in database queries, especially when executing a SELECT statement with an ORDER BY clause. The following is a detailed description of how MySQL implements data sorting:

1. Sorting using indexes

When there is an index on the sort key (the column in the ORDER BY clause) used in a query, MySQL can directly utilize this index to return ordered data without the need for additional sorting operations. This can greatly improve query efficiency because the index itself is ordered.

Index Scanning: If the query can be fully satisfied by an index (i.e., overriding the index), MySQL can return ordered rows of data directly through index scanning without reading the actual rows of data in the table.
Index Coverage Scan: When the query columns and sequences are contained in an index, MySQL can execute the query through the index alone, which is called an index coverage scan.

2. Filesort

If the sort key in the query is not supported by an index, or if an index exists but MySQL decides not to use it (for example, because of index selectivity), MySQL will have to perform a Filesort operation.

In-Memory Sorting: First, MySQL tries to do the sorting in memory. It reads the query result set, stores them in memory, and sorts them using a sorting algorithm (e.g., quick sort, merge sort, etc.).
Disk Sort: If the amount of data required for a sort operation exceeds MySQL's sort buffer size (sort_buffer_size), MySQL will have to break the data into multiple chunks, sort each chunk, and then merge the sorted chunks into an ordered result set. This process will write the data to temporary files and may require multiple disk I/O operations, which can lead to performance degradation.

3. Sorting algorithms

MySQL may use a variety of sorting algorithms when performing sorting, depending on the amount of data, available memory, and other factors. Commonly used sorting algorithms include quick sort, merge sort, and others.

4. Optimizing sorting operations

In order to improve the efficiency of the sorting operation, some of the following optimizations can be taken:

Create appropriate indexes: Make sure you create indexes on columns that are frequently used for sorting.
Adjusting the sort buffer size: by adjusting thesort_buffer_size configuration item to allocate more memory for sort operations to reduce disk I/O.
Optimize queries: try to enable queries to utilize indexes and avoid full table scans.
Analyzing Queries with EXPLAIN: Use the EXPLAIN statement to view a query's execution plan to see how MySQL executes the query, including whether indexes are used and whether file sorting is performed.

With these methods, you can effectively improve the performance of MySQL sort operations.

2. Answers of successful interviewers

Interview Answer Ideas:
the essence of an interview:Not answering questions point to point,Rather, the interviewer presents a topic by(concern),Access to the interviewer's thinking and skill level,
So be sure to reflect the thought process when answering the questions(Includes how to think and development experience)and technical depth;
It can be centered around the following3One Direction Answers
1.Answering basic definitions
2.Tell us how it is used in actual production
3.Leads to an in-depth discussion of the technical points in which you specialize

basic definition

From the sql level then the implementation of sorting is in the order by field, ascending or descending, the sorting is done in the order by field, ascending or descending.

The mysql service relies on sorting algorithms and indexes to implement this sorting function.

actual production

In practice, we usually use the primary key or creation time to sort, especially for tables with a large amount of data.

It is generally not recommended to sort by fields that change frequently, such as update time.

Why? This brings us to the effect on modifications and additions when an index is created for a field; the

We all know that indexes improve query speed, but the efficiency will be reduced when adding and modifying.

The actual development of sorted fields will generally require the creation of indexes.

index sorting

In the case of indexed sorting, it is divided into two cases

1. Index Scanning

2. Index coverage scanning

Index scanning, which sorts by index and then reads the actual rows in the table.

Index Override Scanning: When the query columns and rows are contained in an index, MySQL can execute the query through the index alone without reading the actual rows of data in the table, which is much more efficient.

Therefore, the actual development we generally try to take only the need to return to the field, do not gulp each column are returned, so not only do not cover the index, and may increase disk IO.

File sorting (filesort)

If it is not the index field sorting, in fact, is often referred to as file sorting (filesort), this time also be divided into two cases

1. Memory Sorting

2. Disk Sorting

memory sortAs the name suggests is to read the data into memory for sorting, using sorting algorithms for sorting, but if the amount of data is large, the memory can not be put, what will happen? Memory overflow, error.

Of course not, the mysql service isn't that stupid, when it runs out of memory, it dumps it to disk for sorting.

Disk Sorting: If the amount of data required for a sort operation exceeds MySQL's sort_buffer_size, MySQL will have to split the data into multiple chunks and sort each chunk.

The sorted blocks are then merged into an ordered result set. This process will write data to a temporary file and may require multiple disk I/O operations, which can lead to performance degradation.

This shows that the setting of the sort buffer parameter is an important part of mysql tuning

Of course, these are theories, the actual development of a sorted sql with the execution of slow, we should use explain to see the specific causes

Remarks.

is a very important tool to optimize sql, this must be able to ....

2. On the sorting algorithm, if you have studied before, you can explore a little deeper

3. Summary & Comments

2 ways to answer above.

The first, which is more of a point-to-point answer, is similar to the way we answer question papers when we are studying; the second, which is more of a point-to-point answer, is similar to the way we answer question papers when we are studying.

(b) The second, which is more oriented towards combining pre-theoretical answers with practical development and focuses more on the thought process that led to the conclusions.

If you were the interviewer, which answer would you find more favorable?

Feel free to give your opinion in the comments section!