What is a parse call in MySQL?

What is a parse call in MySQL?


In MySQL, a parse call refers to the process of analyzing a SQL statement and preparing it for execution. When a client sends a SQL statement to the MySQL server, the server parses the statement to check its syntax and validity, and to create an execution plan.

The parse call is performed by the MySQL parser, which is responsible for validating the syntax of the SQL statement, checking the privileges of the user who sent the statement, and determining the best execution plan for the statement. The parser generates an internal representation of the statement, known as a parse tree, which is used to execute the statement.

The parse call is a relatively expensive operation, as it requires the MySQL server to allocate memory and perform complex analysis. Therefore, if the same SQL statement is executed multiple times, it’s more efficient to use prepared statements, which allows the MySQL server to reuse the parse tree and execution plan, avoiding the need to reparse the statement.

In summary, a parse call in MySQL refers to the process of analyzing a SQL statement and preparing it for execution. The parse call is performed by the MySQL parser which is responsible for validating the syntax of the SQL statement, checking the privileges of the user who sent the statement, and determining the best execution plan for the statement. The parse call is a relatively expensive operation, therefore, if the same SQL statement is executed multiple times, it’s more efficient to use prepared statements, which allows the MySQL server to reuse the parse tree and execution plan, avoiding the need to reparse the statement.

How does cursor sharing happen in MySQL?

Cursor sharing in MySQL refers to the process of reusing the same execution plan and cursor for multiple, similar SQL statements. This allows the MySQL server to avoid the cost of parsing and optimizing each individual statement, improving performance.

When a client sends a SQL statement to the MySQL server, the server checks the statement’s text against a cache of previously parsed and optimized statements. If the statement text matches an existing statement in the cache, the MySQL server reuses the cached execution plan and cursor for the new statement.

If the statement text does not match any existing statement in the cache, the MySQL server parses and optimizes the statement as usual, and then adds it to the cache for future use.

Cursor sharing can be enabled or disabled at the session or global level. It can be enabled by setting the session variable cursor_sharing to ‘force’ or ‘similar’.

MySQL also uses a technique called ‘SQL rewrite’ which is a process of rewriting the SQL statement in order to make it more similar to other statements in the cache, thus increasing the chances of finding a match.

In summary, cursor sharing in MySQL refers to the process of reusing the same execution plan and cursor for multiple, similar SQL statements. This allows the MySQL server to avoid the cost of parsing and optimizing each individual statement, improving performance. Cursor sharing can be enabled or disabled at the session or global level, and MySQL also uses a technique called ‘SQL rewrite’ which is a process of rewriting the SQL statement in order to make it more similar to other statements in the cache, thus increasing the chances of finding a match.

How is cursor caching implemented in MySQL?

Cursor caching, also known as cursor pooling, in MySQL is implemented by the server to minimize the overhead of creating and closing cursors. When a client sends a SQL statement to the MySQL server, the server first checks if a cursor already exists in the cursor cache that can be reused for that statement. If a reusable cursor is found, the server will use it, otherwise, it will create a new cursor, and add it to the cursor cache.

MySQL uses a Least Recently Used (LRU) algorithm to manage the cursor cache. When the cursor cache reaches its maximum size, the least recently used cursor is evicted from the cache to make room for a new cursor.

The cursor cache size can be configured at the server level. The default value is usually 100, but it can be modified by setting the max_open_cursors system variable.

It’s also worth noting that, cursor caching is different from statement caching, which is the process of caching the execution plan of a SQL statement to avoid reparsing and optimizing the statement. Statement caching and statement caching are used together to improve the performance of the MySQL server.

In summary, Cursor caching, also known as cursor pooling, in MySQL is implemented by the server to minimize the overhead of creating and closing cursors. MySQL uses a Least Recently Used (LRU) algorithm to manage the cursor cache, and the cursor cache size can be configured at the server level. Cursor caching is different from statement caching which is the process of caching the execution plan of a SQL statement to avoid reparsing and optimizing the statement.

About Shiv Iyer 72 Articles
Open Source Database Systems Engineer with a deep understanding of Optimizer Internals, Performance Engineering, Scalability and Data SRE. Shiv currently is the Founder, Investor, Board Member and CEO of multiple Database Systems Infrastructure Operations companies in the Transaction Processing Computing and ColumnStores ecosystem. He is also a frequent speaker in open source software conferences globally.

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