SQLite

  setLikeOptFlag(db, "like", 
      caseSensitive ? (SQLITE_FUNC_LIKE | SQLITE_FUNC_CASE) : SQLITE_FUNC_LIKE);
}

/*
** pExpr points to an expression which implements a function.  If
** it is appropriate to apply the LIKE optimization to that function
** then set aWc[0] through aWc[2] to the wildcard characters and the
** escape character and then return TRUE.  If the function is not a 
** LIKE-style function then return FALSE.
**
** The expression "a LIKE b ESCAPE c" is only considered a valid LIKE
** operator if c is a string literal that is exactly one byte in length.
** That one byte is stored in aWc[3].  aWc[3] is set to zero if there is
** no ESCAPE clause.
**
** *pIsNocase is set to true if uppercase and lowercase are equivalent for
** the function (default for LIKE).  If the function makes the distinction
** between uppercase and lowercase (as does GLOB) then *pIsNocase is set to
** false.
*/
int sqlite3IsLikeFunction(sqlite3 *db, Expr *pExpr, int *pIsNocase, char *aWc){
  FuncDef *pDef;
  int nExpr;
  if( pExpr->op!=TK_FUNCTION || !pExpr->x.pList ){



    return 0;
  }
  assert( !ExprHasProperty(pExpr, EP_xIsSelect) );
  nExpr = pExpr->x.pList->nExpr;
  pDef = sqlite3FindFunction(db, pExpr->u.zToken, nExpr, SQLITE_UTF8, 0);
  if( NEVER(pDef==0) || (pDef->funcFlags & SQLITE_FUNC_LIKE)==0 ){
    return 0;
  }
  if( nExpr<3 ){
    aWc[3] = 0;
  }else{
    Expr *pEscape = pExpr->x.pList->a[2].pExpr;
    char *zEscape;
    if( pEscape->op!=TK_STRING ) return 0;
    zEscape = pEscape->u.zToken;
    if( zEscape[0]==0 || zEscape[1]!=0 ) return 0;
    aWc[3] = zEscape[0];
  }

  /* The memcpy() statement assumes that the wildcard characters are
  ** the first three statements in the compareInfo structure.  The
  ** asserts() that follow verify that assumption
  */
  memcpy(aWc, pDef->pUserData, 3);
  assert( (char*)&likeInfoAlt == (char*)&likeInfoAlt.matchAll );

  if( pExpr->op==TK_IS 
   || pExpr->op==TK_ISNULL || pExpr->op==TK_ISNOT 
   || pExpr->op==TK_NOTNULL || pExpr->op==TK_CASE 
  ){
    pWalker->eCode = 1;
  }else if( pExpr->op==TK_FUNCTION ){
    int d1;
    char d2[4];
    if( 0==sqlite3IsLikeFunction(pWalker->pParse->db, pExpr, &d1, d2) ){
      pWalker->eCode = 1;
    }
  }

  return WRC_Continue;
}

  int *pnoCase      /* True if uppercase is equivalent to lowercase */
){
  const char *z = 0;         /* String on RHS of LIKE operator */
  Expr *pRight, *pLeft;      /* Right and left size of LIKE operator */
  ExprList *pList;           /* List of operands to the LIKE operator */
  int c;                     /* One character in z[] */
  int cnt;                   /* Number of non-wildcard prefix characters */
  char wc[4];                /* Wildcard characters */
  sqlite3 *db = pParse->db;  /* Database connection */
  sqlite3_value *pVal = 0;
  int op;                    /* Opcode of pRight */
  int rc;                    /* Result code to return */

  if( !sqlite3IsLikeFunction(db, pExpr, pnoCase, wc) ){
    return 0;

       || sqlite3ExprAffinity(pLeft)!=SQLITE_AFF_TEXT 
       || IsVirtual(pLeft->pTab)  /* Value might be numeric */
      ){
        sqlite3ValueFree(pVal);
        return 0;
      }
    }

    /* Count the number of prefix characters prior to the first wildcard */
    cnt = 0;
    while( (c=z[cnt])!=0 && c!=wc[0] && c!=wc[1] && c!=wc[2] ){
      cnt++;
      if( c==wc[3] && z[cnt]!=0 ){
        if( z[cnt++]>0xc0 ) while( (z[cnt]&0xc0)==0x80 ){ cnt++; }
      }
    }

    /* The optimization is possible only if (1) the pattern does not begin
    ** with a wildcard and if (2) the non-wildcard prefix does not end with
    ** an (illegal 0xff) character.  The second condition is necessary so
    ** that we can increment the prefix key to find an upper bound for the
    ** range search. 
    */
    if( cnt!=0 && 255!=(u8)z[cnt-1] ){
      Expr *pPrefix;

      /* A "complete" match if the pattern ends with "*" or "%" */
      *pisComplete = c==wc[0] && z[cnt+1]==0;

      /* Get the pattern prefix.  Remove all escapes from the prefix. */
      pPrefix = sqlite3Expr(db, TK_STRING, z);
      if( pPrefix ){
        int iFrom, iTo;
        char *zNew = pPrefix->u.zToken;
        zNew[cnt] = 0;
        for(iFrom=iTo=0; iFrom<cnt; iFrom++){
          if( zNew[iFrom]==wc[3] ) iFrom++;
          zNew[iTo++] = zNew[iFrom];
        }
        zNew[iTo] = 0;
      }
      *ppPrefix = pPrefix;

      /* If the RHS pattern is a bound parameter, make arrangements to
      ** reprepare the statement when that parameter is rebound */
      if( op==TK_VARIABLE ){
        Vdbe *v = pParse->pVdbe;
        sqlite3VdbeSetVarmask(v, pRight->iColumn);
        if( *pisComplete && pRight->u.zToken[1] ){
          /* If the rhs of the LIKE expression is a variable, and the current
          ** value of the variable means there is no need to invoke the LIKE
          ** function, then no OP_Variable will be added to the program.

    PRAGMA case_sensitive_like=on;
    CREATE INDEX i1 ON t1(x);
  }
  queryplan {
    SELECT x FROM t1 WHERE x LIKE 'abc%' ORDER BY 1;
  }
} {abc abcd nosort {} i1}
do_test like-3.3.100.cnt {
  set sqlite_like_count
} 0

# The like optimization works even when the pattern is a bound parameter
#
# Exception: It does not work if sqlite3_prepare() is used instead of
# sqlite3_prepare_v2(), as in that case the statement cannot be reprepared

    set x [lindex [time {
      db one {SELECT 'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaz'LIKE'%a%a%a%a%a%a%a%a%y'}
    }] 0]
    puts -nonewline " ($x ms - want less than 1000) "
    expr {$x<1000}
  } {1}
}

# As of 2017-07-27 (3.21.0) the LIKE optimization works with ESCAPE as
# long as the ESCAPE is a single-byte literal.
#
db close
sqlite3 db :memory:
do_execsql_test like-15.100 {
  CREATE TABLE t15(x TEXT COLLATE nocase, y, PRIMARY KEY(x));
  INSERT INTO t15(x,y) VALUES
    ('abcde',1), ('ab%de',2), ('a_cde',3),
    ('uvwxy',11),('uvwx%',12),('uvwx_',13),
    ('_bcde',21),('%bcde',22),
    ('abcd_',31),('abcd%',32),
    ('ab%xy',41);
  SELECT y FROM t15 WHERE x LIKE 'ab/%d%' ESCAPE '/';
} {2}
do_execsql_test like-15.101 {
  EXPLAIN QUERY PLAN
  SELECT y FROM t15 WHERE x LIKE 'ab/%d%' ESCAPE '/';
} {/SEARCH/}
do_execsql_test like-15.102 {
  EXPLAIN QUERY PLAN
  SELECT y FROM t15 WHERE x LIKE 'ab/%d%' ESCAPE '//';
} {/SCAN/}
do_execsql_test like-15.103 {
  EXPLAIN QUERY PLAN
  SELECT y FROM t15 WHERE x LIKE 'ab/%d%' ESCAPE '';
} {/SCAN/}
do_execsql_test like-15.110 {
  SELECT y FROM t15 WHERE x LIKE 'abcdx%%' ESCAPE 'x';
} {32}
do_execsql_test like-15.111 {
  SELECT y FROM t15 WHERE x LIKE 'abx%%' ESCAPE 'x' ORDER BY +y
} {2 41}
do_execsql_test like-15.112 {
  EXPLAIN QUERY PLAN
  SELECT y FROM t15 WHERE x LIKE 'abx%%' ESCAPE 'x' ORDER BY +y
} {/SEARCH/}
do_execsql_test like-15.120 {
  SELECT y FROM t15 WHERE x LIKE '/%bc%' ESCAPE '/';
} {22}
do_execsql_test like-15.121 {
  EXPLAIN QUERY PLAN
  SELECT y FROM t15 WHERE x LIKE '/%bc%' ESCAPE '/';
} {/SEARCH/}




finish_test