Shulou(Shulou.com)06/01 Report--

Original: http://www.sqlite.org/lang_explain.html

The keyword "EXPLAIN" or the statement (by the phrase) "EXPLAIN QUERY PLAN" can be placed before the SQL statement. (provide the original text about this sentence to facilitate later users to view An SQL statement can be preceded by the keyword "EXPLAIN". Without the EXPLAIN keyword or statement, the SQL statement is no different from the usual query information (Either modification causes the SQL statement to behave as qeruy and to return informationabout how the SQL statement would have operated if the EXPLAIN keyword orphrase had been omitted.)

The output of "EXPLAIN" and "EXPLAIN QUERY PLAN" is mainly used for data exchange analysis and fault finding. The details of the output vary depending on the current version of SQLite. Applications should not use this statement because the results of execution are uncertain and only a small amount of information is recorded (since their exact behavior variable and only partially documented).

Note: in the sqlite3.exe usage tutorial, the basic usage is briefly introduced, and an index is created to show how to analyze the efficiency of the executed SQL.

When the EXPLAIN keyword appears before the SQL statement, the query returns a set of instructions executed by the virtual machine and outputs it to the console. When using EXPLAIN QUERY TEXT, more advanced information is returned, but more detailed information is returned. In fact, the returned information is mainly scanned the whole table, or the index is used to query, of course, the premise is that the index is necessary.

EXPLAIN is executed at run time, not at preparatory time

The EXPLAIN and EXPLAIN QUERY PLAN prefixes affect the ability to run a precompiled declaration that calls sqlite3_step. The process of generating a new precompiled declaration by calling sqlite3_prepare is not affected by EXPLAIN. The exception is that there are special opcodes that prohibit the use of the EXPLAIN QUERY PLAN keyword, which is ultimately the result of performance optimization.

It means that the process of calling the sqlite3_prepare function will not be affected by EXPLAIN.

1) some compilation declarations work on sqlite3_prepare, not on sqlite3_step. These compilation declarations are not affected by EXPLAIN.

2) regardless of whether the EXPLAIN or EXPLAIN QUERY PLAN keyword is specified in the SQL statement, the authorizer callback function will be executed

Welcome to subscribe "Shulou Technology Information " to get latest news, interesting things and hot topics in the IT industry, and controls the hottest and latest Internet news, technology news and IT industry trends.