An SQL statement can be preceded by the keyword "EXPLAIN" or by the phrase "EXPLAIN QUERY PLAN". Either modification causes the SQL statement to behave as a query and to return information about how the SQL statement would have operated if the EXPLAIN keyword or phrase had been omitted.
The output from EXPLAIN and EXPLAIN QUERY PLAN is intended for interactive analysis and troubleshooting only. The details of the output format are subject to change from one release of SQLite to the next. Applications should not use EXPLAIN or EXPLAIN QUERY PLAN since their exact behavior is variable and only partially documented.
When the EXPLAIN keyword appears by itself it causes the statement to behave as a query that returns the sequence of virtual machine instructions it would have used to execute the command had the EXPLAIN keyword not been present. When the EXPLAIN QUERY PLAN phrase appears, the statement returns high-level information regarding the query plan that would have been used.
The EXPLAIN QUERY PLAN command is described in more detail here.
The EXPLAIN and EXPLAIN QUERY PLAN prefixes affect the behavior of running a prepared statement using sqlite3_step(). The process of generating a new prepared statement using sqlite3_prepare() or similar is (mostly) unaffected by EXPLAIN. (The exception to the previous sentence is that some special opcodes used by EXPLAIN QUERY PLAN are omitted when building an EXPLAIN QUERY PLAN prepared statement, as a performance optimization.)
This means that actions that occur during sqlite3_prepare() are unaffected by EXPLAIN.
Some PRAGMA statements do their work during sqlite3_prepare() rather than during sqlite3_step(). Those PRAGMA statements are unaffected by EXPLAIN. They operate the same with or without the EXPLAIN prefix. The set of PRAGMA statements that are unaffected by EXPLAIN can vary from one release to the next. Some PRAGMA statements operate during sqlite3_prepare() depending on their arguments. For consistent results, avoid using EXPLAIN on PRAGMA statements.
The authorizer callback is invoked regardless of the presence of EXPLAIN or EXPLAIN QUERY PLAN.
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