Some questions about gin index

Hi,

When reading the code for Gin Index, there are lots of confusion about
some concepts, so here are some questions in my mind.

1. search mode.

/*
* searchMode settings for extractQueryFn.
*/
#define GIN_SEARCH_MODE_DEFAULT 0
#define GIN_SEARCH_MODE_INCLUDE_EMPTY 1
#define GIN_SEARCH_MODE_ALL 2
#define GIN_SEARCH_MODE_EVERYTHING 3 /* for internal use only */

2.

typedef struct GinScanEntryData
{
..
bool isPartialMatch;
..
}

3. requiredEntries / additionalEntries

typedef struct GinScanKeyData
{
/*
* At least one of the entries in requiredEntries must be present for a
* tuple to match the overall qual.
*
* additionalEntries contains entries that are needed by the consistent
* function to decide if an item matches, but are not sufficient to
* satisfy the qual without entries from requiredEntries.
*/
GinScanEntry *requiredEntries;
int nrequired;
GinScanEntry *additionalEntries;
int nadditional;
..
}

For all the 3 questions, the general question is what is X and how does
it play its role? I have read the gin/README before asking, so any help
is appreciated.

--
Best Regards
Andy Fan