SQLite

  if( sqlite3AuthCheck(pParse, SQLITE_TRANSACTION, "BEGIN", 0, 0) ){
    return;
  }
  v = sqlite3GetVdbe(pParse);
  if( !v ) return;
  if( type!=TK_DEFERRED ){
    for(i=0; i<db->nDb; i++){
      sqlite3VdbeAddOp3(v, OP_Transaction, i, (type==TK_EXCLUSIVE)+1, 1);
      sqlite3VdbeUsesBtree(v, i);
    }
  }
  sqlite3VdbeAddOp0(v, OP_AutoCommit);
}

/*

/* Opcode: Transaction P1 P2 P3 P4 P5
**
** Begin a transaction on database P1 if a transaction is not already
** active.
** If P2 is non-zero, then a write-transaction is started, or if a 
** read-transaction is already active, it is upgraded to a write-transaction.
** If P2 is zero, then a read-transaction is started.  The 0x02 bit of P2
** is set for an exclusive transaction.
**
** P1 is the index of the database file on which the transaction is
** started.  Index 0 is the main database file and index 1 is the
** file used for temporary tables.  Indices of 2 or more are used for
** attached databases.
**
** If a write-transaction is started and the Vdbe.usesStmtJournal flag is
** true (this flag is set if the Vdbe may modify more than one row and may
** throw an ABORT exception), a statement transaction may also be opened.
** More specifically, a statement transaction is opened iff the database
** connection is currently not in autocommit mode, or if there are other
** active statements. A statement transaction allows the changes made by this
** VDBE to be rolled back after an error without having to roll back the
** entire transaction. If no error is encountered, the statement transaction
** will automatically commit when the VDBE halts.
**
** If P5==1 then this opcode also checks the schema cookie against P3
** and the schema generation counter against P4.
** The cookie changes its value whenever the database schema changes.
** This operation is used to detect when that the cookie has changed
** and that the current process needs to reread the schema.  If the schema
** cookie in P3 differs from the schema cookie in the database header or
** if the schema generation counter in P4 differs from the current
** generation counter, then an SQLITE_SCHEMA error is raised and execution
** halts.  The sqlite3_step() wrapper function might then reprepare the
** statement and rerun it from the beginning.
**
** If P5==0 and P3==1 this is the transaction started by a BEGIN 
** IMMEDIATE or BEGIN EXCLUSIVE command.
*/
case OP_Transaction: {
  Btree *pBt;
  int iMeta;
  int iGen;

  assert( p->bIsReader );
  assert( p->readOnly==0 || pOp->p2==0 );
  assert( pOp->p1>=0 && pOp->p1<db->nDb );
  assert( DbMaskTest(p->btreeMask, pOp->p1) );
  if( pOp->p2 ){
    if( (db->flags & SQLITE_QueryOnly)!=0 ){
      rc = SQLITE_READONLY;
      goto abort_due_to_error;
    }
    if( (db->flags & SQLITE_RequireTxn)!=0 
     && db->autoCommit
     && (pOp->p5!=0 || pOp->p3!=1)
    ){
      sqlite3VdbeError(p, "cannot write outside of a transaction");
      rc = SQLITE_ERROR;

      goto abort_due_to_error;
    }
  }
  pBt = db->aDb[pOp->p1].pBt;

  if( pBt ){
    rc = sqlite3BtreeBeginTrans(pBt, pOp->p2);
    testcase( rc==SQLITE_BUSY_SNAPSHOT );
    testcase( rc==SQLITE_BUSY_RECOVERY );

    */
    sqlite3BtreeGetMeta(pBt, BTREE_SCHEMA_VERSION, (u32 *)&iMeta);
    iGen = db->aDb[pOp->p1].pSchema->iGeneration;
  }else{
    iGen = iMeta = 0;
  }
  assert( pOp->p5==0 || pOp->p4type==P4_INT32 );
  if( pOp->p5==1 && (iMeta!=pOp->p3 || iGen!=pOp->p4.i) ){
    sqlite3VdbeError(p, "database schema has changed");
    /* If the schema-cookie from the database file matches the cookie 
    ** stored with the in-memory representation of the schema, do
    ** not reload the schema from the database file.
    **
    ** If virtual-tables are in use, this is not just an optimization.
    ** Often, v-tables store their data in other SQLite tables, which