** three columns in the table.  In the Index structure describing
** Ex2, nColumn==2 since 2 of the 3 columns of Ex1 are indexed.
** The value of aiColumn is {2, 0}.  aiColumn[0]==2 because the 
** first column to be indexed (c3) has an index of 2 in Ex1.aCol[].
** The second column to be indexed (c1) has an index of 0 in
** Ex1.aCol[], hence Ex2.aiColumn[1]==0.
**
** The Index.onError field determines whether or not the indexed columns
** must be unique and what to do if they are not.  When Index.onError=OE_None,
** it means this is not a unique index.  Otherwise it is a unique index
** and the value of Index.onError indicate the which conflict resolution 
** algorithm to employ whenever an attempt is made to insert a non-unique
** element.
*/
struct Index {
  char *zName;     /* Name of this index */
  int nColumn;     /* Number of columns in the table used by this index */
  int *aiColumn;   /* Which columns are used by this index.  1st is 0 */
  unsigned *aiRowEst; /* Result of ANALYZE: Est. rows selected by each column */
  Table *pTable;   /* The SQL table being indexed */
  int tnum;        /* Page containing root of this index in database file */
  u8 onError;      /* OE_Abort, OE_Ignore, OE_Replace, or OE_None */
  u8 autoIndex;    /* True if is automatically created (ex: by UNIQUE) */
  char *zColAff;   /* String defining the affinity of each column */
  Index *pNext;    /* The next index associated with the same table */
  Schema *pSchema; /* Schema containing this index */
  u8 *aSortOrder;  /* Array of size Index.nColumn. True==DESC, False==ASC */
  char **azColl;   /* Array of collation sequence names for index */
};

/*
** Each token coming out of the lexer is an instance of
** this structure.  Tokens are also used as part of an expression.
**
** Note if Token.z==0 then Token.dyn and Token.n are undefined and
** may contain random values.  Do not make any assuptions about Token.dyn
** and Token.n when Token.z==0.
*/
struct Token {
  const unsigned char *z; /* Text of the token.  Not NULL-terminated! */
  unsigned dyn  : 1;      /* True for malloced memory, false for static */
  unsigned n    : 31;     /* Number of characters in this token */
};

/*
** An instance of this structure contains information needed to generate
** code for a SELECT that contains aggregate functions.
**
** If Expr.op==TK_AGG_COLUMN or TK_AGG_FUNCTION then Expr.pAggInfo is a
** pointer to this structure.  The Expr.iColumn field is the index in
** AggInfo.aCol[] or AggInfo.aFunc[] of information needed to generate
** code for that node.
**
** AggInfo.pGroupBy and AggInfo.aFunc.pExpr point to fields within the
** original Select structure that describes the SELECT statement.  These
** fields do not need to be freed when deallocating the AggInfo structure.
*/
struct AggInfo {
  u8 directMode;          /* Direct rendering mode means take data directly
                          ** from source tables rather than from accumulators */
  u8 useSortingIdx;       /* In direct mode, reference the sorting index rather
                          ** than the source table */
  int sortingIdx;         /* Cursor number of the sorting index */
  ExprList *pGroupBy;     /* The group by clause */
  int nSortingColumn;     /* Number of columns in the sorting index */
  struct AggInfo_col {    /* For each column used in source tables */
    int iTable;              /* Cursor number of the source table */
    int iColumn;             /* Column number within the source table */
    int iSorterColumn;       /* Column number in the sorting index */
    int iMem;                /* Memory location that acts as accumulator */
    Expr *pExpr;             /* The original expression */
  } *aCol;
  int nColumn;            /* Number of used entries in aCol[] */
  int nColumnAlloc;       /* Number of slots allocated for aCol[] */
  int nAccumulator;       /* Number of columns that show through to the output.
                          ** Additional columns are used only as parameters to
                          ** aggregate functions */
  struct AggInfo_func {   /* For each aggregate function */
    Expr *pExpr;             /* Expression encoding the function */
    FuncDef *pFunc;          /* The aggregate function implementation */
    int iMem;                /* Memory location that acts as accumulator */
    int iDistinct;           /* Ephermeral table used to enforce DISTINCT */
  } *aFunc;
  int nFunc;              /* Number of entries in aFunc[] */
  int nFuncAlloc;         /* Number of slots allocated for aFunc[] */
};

/*
** Each node of an expression in the parse tree is an instance
** of this structure.
**
** Expr.op is the opcode.  The integer parser token codes are reused
** as opcodes here.  For example, the parser defines TK_GE to be an integer
** code representing the ">=" operator.  This same integer code is reused
** to represent the greater-than-or-equal-to operator in the expression
** tree.
**
** Expr.pRight and Expr.pLeft are subexpressions.  Expr.pList is a list
** of argument if the expression is a function.
**
** Expr.token is the operator token for this node.  For some expressions
** that have subexpressions, Expr.token can be the complete text that gave
** rise to the Expr.  In the latter case, the token is marked as being
** a compound token.
**
** An expression of the form ID or ID.ID refers to a column in a table.
** For such expressions, Expr.op is set to TK_COLUMN and Expr.iTable is
** the integer cursor number of a VDBE cursor pointing to that table and
** Expr.iColumn is the column number for the specific column.  If the
** expression is used as a result in an aggregate SELECT, then the
** value is also stored in the Expr.iAgg column in the aggregate so that
** it can be accessed after all aggregates are computed.
**
** If the expression is a function, the Expr.iTable is an integer code
** representing which function.  If the expression is an unbound variable
** marker (a question mark character '?' in the original SQL) then the
** Expr.iTable holds the index number for that variable.
**
** If the expression is a subquery then Expr.iColumn holds an integer
** register number containing the result of the subquery.  If the
** subquery gives a constant result, then iTable is -1.  If the subquery
** gives a different answer at different times during statement processing
** then iTable is the address of a subroutine that computes the subquery.
**
** The Expr.pSelect field points to a SELECT statement.  The SELECT might
** be the right operand of an IN operator.  Or, if a scalar SELECT appears
** in an expression the opcode is TK_SELECT and Expr.pSelect is the only
** operand.
**
** If the Expr is of type OP_Column, and the table it is selecting from
** is a disk table or the "old.*" pseudo-table, then pTab points to the
** corresponding table definition.
*/
struct Expr {
  u8 op;                 /* Operation performed by this node */
  char affinity;         /* The affinity of the column or 0 if not a column */
  u16 flags;             /* Various flags.  See below */
  CollSeq *pColl;        /* The collation type of the column or 0 */
  Expr *pLeft, *pRight;  /* Left and right subnodes */
  ExprList *pList;       /* A list of expressions used as function arguments
                         ** or in "<expr> IN (<expr-list)" */
  Token token;           /* An operand token */
  Token span;            /* Complete text of the expression */
  int iTable, iColumn;   /* When op==TK_COLUMN, then this expr node means the
                         ** iColumn-th field of the iTable-th table. */
  AggInfo *pAggInfo;     /* Used by TK_AGG_COLUMN and TK_AGG_FUNCTION */
  int iAgg;              /* Which entry in pAggInfo->aCol[] or ->aFunc[] */
  int iRightJoinTable;   /* If EP_FromJoin, the right table of the join */
  Select *pSelect;       /* When the expression is a sub-select.  Also the
                         ** right side of "<expr> IN (<select>)" */
  Table *pTab;           /* Table for OP_Column expressions. */
  Schema *pSchema;
};

/*
** The following are the meanings of bits in the Expr.flags field.
*/
#define EP_FromJoin     0x01  /* Originated in ON or USING clause of a join */
#define EP_Agg          0x02  /* Contains one or more aggregate functions */
#define EP_Resolved     0x04  /* IDs have been resolved to COLUMNs */
#define EP_Error        0x08  /* Expression contains one or more errors */
#define EP_Distinct     0x10  /* Aggregate function with DISTINCT keyword */
#define EP_VarSelect    0x20  /* pSelect is correlated, not constant */
#define EP_Dequoted     0x40  /* True if the string has been dequoted */
#define EP_InfixFunc    0x80  /* True for an infix function: LIKE, GLOB, etc */

/*
** These macros can be used to test, set, or clear bits in the 
** Expr.flags field.
*/
#define ExprHasProperty(E,P)     (((E)->flags&(P))==(P))
#define ExprHasAnyProperty(E,P)  (((E)->flags&(P))!=0)
#define ExprSetProperty(E,P)     (E)->flags|=(P)
#define ExprClearProperty(E,P)   (E)->flags&=~(P)

/*
** A list of expressions.  Each expression may optionally have a
** name.  An expr/name combination can be used in several ways, such
** as the list of "expr AS ID" fields following a "SELECT" or in the
** list of "ID = expr" items in an UPDATE.  A list of expressions can
** also be used as the argument to a function, in which case the a.zName
** field is not used.
*/
struct ExprList {
  int nExpr;             /* Number of expressions on the list */
  int nAlloc;            /* Number of entries allocated below */
  int iECursor;          /* VDBE Cursor associated with this ExprList */
  struct ExprList_item {
    Expr *pExpr;           /* The list of expressions */
    char *zName;           /* Token associated with this expression */
    u8 sortOrder;          /* 1 for DESC or 0 for ASC */
    u8 isAgg;              /* True if this is an aggregate like count(*) */
    u8 done;               /* A flag to indicate when processing is finished */
  } *a;                  /* One entry for each expression */
};

/*
** An instance of this structure can hold a simple list of identifiers,
** such as the list "a,b,c" in the following statements:
**
**      INSERT INTO t(a,b,c) VALUES ...;
**      CREATE INDEX idx ON t(a,b,c);
**      CREATE TRIGGER trig BEFORE UPDATE ON t(a,b,c) ...;
**
** The IdList.a.idx field is used when the IdList represents the list of
** column names after a table name in an INSERT statement.  In the statement
**
**     INSERT INTO t(a,b,c) ...
**
** If "a" is the k-th column of table "t", then IdList.a[0].idx==k.
*/
struct IdList {
  struct IdList_item {
    char *zName;      /* Name of the identifier */
    int idx;          /* Index in some Table.aCol[] of a column named zName */
  } *a;
  int nId;         /* Number of identifiers on the list */
  int nAlloc;      /* Number of entries allocated for a[] below */
};

/*
** The bitmask datatype defined below is used for various optimizations.
*/
typedef unsigned int Bitmask;

/*
** The following structure describes the FROM clause of a SELECT statement.
** Each table or subquery in the FROM clause is a separate element of
** the SrcList.a[] array.
**
** With the addition of multiple database support, the following structure
** can also be used to describe a particular table such as the table that
** is modified by an INSERT, DELETE, or UPDATE statement.  In standard SQL,
** such a table must be a simple name: ID.  But in SQLite, the table can
** now be identified by a database name, a dot, then the table name: ID.ID.
*/
struct SrcList {
  i16 nSrc;        /* Number of tables or subqueries in the FROM clause */
  i16 nAlloc;      /* Number of entries allocated in a[] below */
  struct SrcList_item {
    char *zDatabase;  /* Name of database holding this table */
    char *zName;      /* Name of the table */
    char *zAlias;     /* The "B" part of a "A AS B" phrase.  zName is the "A" */
    Table *pTab;      /* An SQL table corresponding to zName */
    Select *pSelect;  /* A SELECT statement used in place of a table name */
    u8 isPopulated;   /* Temporary table associated with SELECT is populated */
    u8 jointype;      /* Type of join between this table and the next */
    i16 iCursor;      /* The VDBE cursor number used to access this table */
    Expr *pOn;        /* The ON clause of a join */
    IdList *pUsing;   /* The USING clause of a join */
    Bitmask colUsed;  /* Bit N (1<<N) set if column N or pTab is used */
  } a[1];             /* One entry for each identifier on the list */
};

/*
** Permitted values of the SrcList.a.jointype field
*/
#define JT_INNER     0x0001    /* Any kind of inner or cross join */
#define JT_CROSS     0x0002    /* Explicit use of the CROSS keyword */
#define JT_NATURAL   0x0004    /* True for a "natural" join */
#define JT_LEFT      0x0008    /* Left outer join */
#define JT_RIGHT     0x0010    /* Right outer join */
#define JT_OUTER     0x0020    /* The "OUTER" keyword is present */
#define JT_ERROR     0x0040    /* unknown or unsupported join type */

/*
** For each nested loop in a WHERE clause implementation, the WhereInfo
** structure contains a single instance of this structure.  This structure
** is intended to be private the the where.c module and should not be
** access or modified by other modules.
**
** The pIdxInfo and pBestIdx fields are used to help pick the best
** index on a virtual table.  The pIdxInfo pointer contains indexing
** information for the i-th table in the FROM clause before reordering.
** All the pIdxInfo pointers are freed by whereInfoFree() in where.c.
** The pBestIdx pointer is a copy of pIdxInfo for the i-th table after
** FROM clause ordering.  This is a little confusing so I will repeat
** it in different words.  WhereInfo.a[i].pIdxInfo is index information 
** for WhereInfo.pTabList.a[i].  WhereInfo.a[i].pBestInfo is the
** index information for the i-th loop of the join.  pBestInfo is always
** either NULL or a copy of some pIdxInfo.  So for cleanup it is 
** sufficient to free all of the pIdxInfo pointers.
** 
*/
struct WhereLevel {
  int iFrom;            /* Which entry in the FROM clause */