ScanKeyData

ScanKeyData

翻譯

ScanKey描述了表或者索引以及常量之間比較操作的應用。 當它是ScanKeys的一部分時，它默認爲AND。如果是一個二進制操作時，索引列爲左操作數
（數據結構也可以支持一元索引操作符，在這種情況下，sk_argument會被省略，但是它現在沒有被實現）
對於一個索引掃描來說，sk_strategy和sk_subtype必須被設置爲正確。但是在一個heap掃描中使用ScanKye時，這兩個參數不會被使用，他們可能會被設置爲InvalidStrategy和InvalidOid。
如果操作運算是collation-sensitive時，sk_collation 必須被正確設置
在 “column op ANY(ARRAY[…])”這種情況下，ScanKey也可以描述爲ScalarArrayOpExpr，它被SK_SEARCHARRAY標識符標識
sk_argument 不是運算符的右側參數類型的值，而是這樣值的數組，並且每個元素比較要用OR連接起來。
ScanKey也可以描述 “column IS NULL” 或者 “column IS NOT NULL”，
這些情況下會通過 SK_SEARCHNULL 和SK_SEARCHNOTNULL 各自標識
參數爲null時 sk_strategy, sk_subtype, sk_collation, 以及sk_func 經常不被使用（除非被 index AM設置)。
只有在索引掃描情況下 SK_SEARCHARRAY, SK_SEARCHNULL 和 SK_SEARCHNOTNULL 才被支持
而且並不是所有的 index AMs 都支持他們,只有分別設置amsearcharray 或者 amsearchnulls時。.
ScanKey 也可以描述一個順序操作符，例如 “ORDER BY indexedcol op constant”的調用，除了（通常）不產生bool運算符，它看起來跟比較運算符是相同的。我們會標識ScanKeys 爲SK_ORDER_BY，但是這裏不能使用 SK_SEARCHARRAY, SK_SEARCHNULL, SK_SEARCHNOTNULL
注意：
一些情況下，ScanKeys 被用來方便的調用程序支持的函數，在這種情況下sk_strategy/sk_subtype 是沒有意義的，（但是sk_collation 可以有)
並且 sk_func 可能會引用一些函數，返回不是bool的其他值。

typedef struct ScanKeyData
{
    int         sk_flags;           /* 標識符 */
    AttrNumber  sk_attno;           /* 表或者索引列 */
    StrategyNumber sk_strategy;     /* 操作符策略值 */
    Oid         sk_subtype;         /* strategy subtype */
    Oid         sk_collation;       /* 如果需要，會使用校驗值 */
    FmgrInfo    sk_func;            /* 查找函數調用 */
    Datum       sk_argument;        /* 比較的數據 */
} ScanKeyData;

typedef ScanKeyData *ScanKey;

原版

/*
 * A ScanKey represents the application of a comparison operator between
 * a table or index column and a constant.  When it's part of an array of
 * ScanKeys, the comparison conditions are implicitly ANDed.  The index
 * column is the left argument of the operator, if it's a binary operator.
 * (The data structure can support unary indexable operators too; in that
 * case sk_argument would go unused.  This is not currently implemented.)
 *
 * For an index scan, sk_strategy and sk_subtype must be set correctly for
 * the operator.  When using a ScanKey in a heap scan, these fields are not
 * used and may be set to InvalidStrategy/InvalidOid.
 *
 * If the operator is collation-sensitive, sk_collation must be set
 * correctly as well.
 *
 * A ScanKey can also represent a ScalarArrayOpExpr, that is a condition
 * "column op ANY(ARRAY[...])".  This is signaled by the SK_SEARCHARRAY
 * flag bit.  The sk_argument is not a value of the operator's right-hand
 * argument type, but rather an array of such values, and the per-element
 * comparisons are to be ORed together.
 *
 * A ScanKey can also represent a condition "column IS NULL" or "column
 * IS NOT NULL"; these cases are signaled by the SK_SEARCHNULL and
 * SK_SEARCHNOTNULL flag bits respectively.  The argument is always NULL,
 * and the sk_strategy, sk_subtype, sk_collation, and sk_func fields are
 * not used (unless set by the index AM).
 *
 * SK_SEARCHARRAY, SK_SEARCHNULL and SK_SEARCHNOTNULL are supported only
 * for index scans, not heap scans; and not all index AMs support them,
 * only those that set amsearcharray or amsearchnulls respectively.
 *
 * A ScanKey can also represent an ordering operator invocation, that is
 * an ordering requirement "ORDER BY indexedcol op constant".  This looks
 * the same as a comparison operator, except that the operator doesn't
 * (usually) yield boolean.  We mark such ScanKeys with SK_ORDER_BY.
 * SK_SEARCHARRAY, SK_SEARCHNULL, SK_SEARCHNOTNULL cannot be used here.
 *
 * Note: in some places, ScanKeys are used as a convenient representation
 * for the invocation of an access method support procedure.  In this case
 * sk_strategy/sk_subtype are not meaningful (but sk_collation can be); and
 * sk_func may refer to a function that returns something other than boolean.
 */
typedef struct ScanKeyData
{
    int         sk_flags;           /* flags, see below */
    AttrNumber  sk_attno;           /* table or index column number */
    StrategyNumber sk_strategy;     /* operator strategy number */
    Oid         sk_subtype;         /* strategy subtype */
    Oid         sk_collation;       /* collation to use, if needed */
    FmgrInfo    sk_func;            /* lookup info for function to call */
    Datum       sk_argument;        /* data to compare */
} ScanKeyData;

typedef ScanKeyData *ScanKey;