SQLite

  ExprList *pChanges,    /* Things to be changed */
  Expr *pWhere,          /* The WHERE clause.  May be null */
  int onError,           /* How to handle constraint errors */
  ExprList *pOrderBy,    /* ORDER BY clause. May be null */
  Expr *pLimit,          /* LIMIT clause. May be null */
  Upsert *pUpsert        /* ON CONFLICT clause, or null */
){
  int i, j, k;           /* Loop counters */
  Table *pTab;           /* The table to be updated */
  int addrTop = 0;       /* VDBE instruction address of the start of the loop */
  WhereInfo *pWInfo;     /* Information about the WHERE clause */
  Vdbe *v;               /* The virtual database engine */
  Index *pIdx;           /* For looping over indices */
  Index *pPk;            /* The PRIMARY KEY index for WITHOUT ROWID tables */
  int nIdx;              /* Number of indices that need updating */

      if( sqlite3StrICmp(pTab->aCol[j].zName, pChanges->a[i].zName)==0 ){
        if( j==pTab->iPKey ){
          chngRowid = 1;
          pRowidExpr = pChanges->a[i].pExpr;
        }else if( pPk && (pTab->aCol[j].colFlags & COLFLAG_PRIMKEY)!=0 ){
          chngPk = 1;
        }
#ifndef SQLITE_OMIT_GENERATED_COLUMNS
        else if( pTab->aCol[j].colFlags & (COLFLAG_STORED|COLFLAG_VIRTUAL) ){
          sqlite3ErrorMsg(pParse, 
             "cannot UPDATE generated column \"%s\"",
             pTab->aCol[j].zName);
          goto update_cleanup;
        }
#endif
        aXRef[j] = i;
        break;
      }
    }
    if( j>=pTab->nCol ){
      if( pPk==0 && sqlite3IsRowid(pChanges->a[i].zName) ){
        j = -1;

  /* Compute the old pre-UPDATE content of the row being changed, if that
  ** information is needed */
  if( chngPk || hasFK || pTrigger ){
    u32 oldmask = (hasFK ? sqlite3FkOldmask(pParse, pTab) : 0);
    oldmask |= sqlite3TriggerColmask(pParse, 
        pTrigger, pChanges, 0, TRIGGER_BEFORE|TRIGGER_AFTER, pTab, onError
    );
    for(i=0, k=regOld; i<pTab->nCol; i++, k++){
      u32 colFlags = pTab->aCol[i].colFlags;
      if( colFlags & COLFLAG_VIRTUAL ){
        k--;
        continue;
      }
      if( oldmask==0xffffffff
       || (i<32 && (oldmask & MASKBIT32(i))!=0)
       || (colFlags & COLFLAG_PRIMKEY)!=0
      ){
        testcase(  oldmask!=0xffffffff && i==31 );
        sqlite3ExprCodeGetColumnOfTable(pParse, pTab, iDataCur, i, k);
      }else{
        sqlite3VdbeAddOp2(v, OP_Null, 0, k);
      }
    }
    if( chngRowid==0 && pPk==0 ){
      sqlite3VdbeAddOp2(v, OP_Copy, regOldRowid, regNewRowid);
    }
  }

  ** the database after the BEFORE triggers are fired anyway (as the trigger 
  ** may have modified them). So not loading those that are not going to
  ** be used eliminates some redundant opcodes.
  */
  newmask = sqlite3TriggerColmask(
      pParse, pTrigger, pChanges, 1, TRIGGER_BEFORE, pTab, onError
  );
  for(i=0, k=regNew; i<pTab->nCol; i++, k++){
    if( i==pTab->iPKey ){
      sqlite3VdbeAddOp2(v, OP_Null, 0, k);
    }else if( (pTab->aCol[i].colFlags & COLFLAG_VIRTUAL)!=0 ){
      k--;
    }else{
      j = aXRef[i];
      if( j>=0 ){
        sqlite3ExprCode(pParse, pChanges->a[j].pExpr, k);
      }else if( 0==(tmask&TRIGGER_BEFORE) || i>31 || (newmask & MASKBIT32(i)) ){
        /* This branch loads the value of a column that will not be changed 
        ** into a register. This is done if there are no BEFORE triggers, or
        ** if there are one or more BEFORE triggers that use this value via
        ** a new.* reference in a trigger program.
        */
        testcase( i==31 );
        testcase( i==32 );
        sqlite3ExprCodeGetColumnOfTable(pParse, pTab, iDataCur, i, k);
      }else{
        sqlite3VdbeAddOp2(v, OP_Null, 0, k);
      }
    }
  }

  /* Fire any BEFORE UPDATE triggers. This happens before constraints are
  ** verified. One could argue that this is wrong.
  */

    ** some of the columns of the row being updated. Load the values for 
    ** all columns not modified by the update statement into their registers
    ** in case this has happened. Only unmodified columns are reloaded.
    ** The values computed for modified columns use the values before the
    ** BEFORE trigger runs.  See test case trigger1-18.0 (added 2018-04-26)
    ** for an example.
    */
    for(i=0, k=regNew; i<pTab->nCol; i++, k++){
      if( pTab->aCol[i].colFlags & COLFLAG_VIRTUAL ){
        k--;
      }else if( aXRef[i]<0 && i!=pTab->iPKey ){
        sqlite3ExprCodeGetColumnOfTable(pParse, pTab, iDataCur, i, k);
      }
    }
  }

  if( !isView ){
    /* Do constraint checks. */
    assert( regOldRowid>0 );