The Bytecode() And Tables_Used() Table-Valued Functions

1. Overview

Bytecode and tables_used are virtual tables built into SQLite that access information about prepared statements. Both bytecode and tables_used operate as table-valued functions. They take a single required argument which is either the text of an SQL statement, or a pointer to an existing prepared statement. The bytecode function returns one row of result for each bytecode operation in the prepared statement. The tables_used function returns one row for each persistent btree (either a table or an index) accessed by the prepared statement.

2. Usage

The bytecode and tables_used tables are only available if SQLite has been compiled with the -DSQLITE_ENABLE_BYTECODE_VTAB compile-time option. The CLI has been compiled that way, and so you can use the standard CLI as a test platform to experiement.

Both virtual tables are read-only eponymous-only virtual tables. You use them by mentioning them directly in the FROM clause of a SELECT statement. They both require a single argument which is the SQL statement to be analyzed. For example:

SELECT * FROM bytecode('SELECT * FROM bytecode(?1)');

The argument can be either the text of an SQL statement, in which case the bytecode (or tables_used) for that statement is returned, or the argument can be a parameter such as ?1 or $stmt that is later bound to a prepared statement object using the sqlite3_bind_pointer() interface. Use a pointer type of "stmt-pointer" for the sqlite3_bind_pointer() interface.

2.1. Schema

The schema of the bytecode table is:

CREATE TABLE bytecode(
  addr INT,
  opcode TEXT,
  p1 INT,
  p2 INT,
  p3 INT,
  p4 TEXT,
  p5 INT,
  comment TEXT,
  subprog TEXT,
  stmt HIDDEN

The first eight columns are the address, opcode, and operands for a single bytecode in the virtual machine that implements the statement. These columns are the same columns output when using EXPLAIN. The bytecode virtual tables shows all operations in the prepared statement, both the main body of the prepared statement and in subprograms used to implement triggers or foreign key actions. The "subprog" field is NULL for the main body of the prepared statement, or is the trigger name or the string "(FK)" for triggers and foreign key actions.

The schema for the tables_used table is:

CREATE TABLE tables_used(
  type TEXT,
  schema TEXT,
  name TEXT,
  wr INT,
  subprog TEXT,
  stmt HIDDEN

The tables_used table is intended to show which btrees of the database file are read or written by a prepared statement, both by the main statement itself but also by related triggers and foreign key actions. The columns are as follows:

  • type → Either "table" or "index", depending on what role the btree is serving.

  • schema → Which database file the btree is located in. This will be "main" for the main database (the usual case), or "temp" for TEMP tables and indexes, or the name assigned to attached databases by the ATTACH statement.

  • name → The name of the table or index

  • wr → 0 if the object is read, 1 if the object is written

  • subprog → The sub-program in which the object is accessed. NULL means the main body of the prepared statement. Otherwise this field is the name of a trigger or "(FK)" for a foreign key action.

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