sqlite查询性能优化（实测项目中节省一半时间）

通过优化读取提升查询效率

只需要在sqlite.c加入如下代码：

#define MAP_BUFFER_LEN 1024 * 1024 * 10
#include <Windows.h>
typedef struct _FILE_MAPING
{
    HANDLE fileH;
    HANDLE mapFileH;
    char* map_buffer;
    SIZE_T mapsize;
    DWORD64 buffer_offset;
    SIZE_T filesize;
    int flag;
}*PFILE_MAPING, FILE_MAPING;
SIZE_T mGetFileSize(PFILE_MAPING p);
    void reles(PFILE_MAPING p)
    {
        if (p->map_buffer)
            UnmapViewOfFile(p->map_buffer);
        if (p->mapFileH)
            CloseHandle(p->mapFileH);
        if (p->fileH)
            CloseHandle(p->fileH);
        memset(p, 0, sizeof(FILE_MAPING));
        free(p);
    }
    int Init_Maping(PFILE_MAPING* p,wchar_t* path, DWORD access, DWORD sharemode, DWORD CreationDisposition, DWORD Flags)
    {
        (*p) = (PFILE_MAPING)malloc(sizeof(FILE_MAPING));
         memset((*p),0, sizeof(FILE_MAPING));
        (*p)->fileH = CreateFileW(path,
            access/*GENERIC_READ | GENERIC_WRITE*/,
            sharemode /*FILE_SHARE_READ| GENERIC_WRITE*/,
            NULL,
            CreationDisposition/*OPEN_EXISTING*/,
            Flags/*FILE_ATTRIBUTE_NORMAL*/,
            NULL);
        if ((*p)->fileH == INVALID_HANDLE_VALUE)
        {
            int erro = GetLastError();
            return 0;
        }
        (*p)->mapFileH = CreateFileMapping((*p)->fileH, NULL, PAGE_READONLY/*PAGE_READWRITE*/, 0, 0, 0);
        if ((*p)->mapFileH == NULL)
        {
            int erro = GetLastError();
            return 0;
        }
        return 1;
    }
    int ReadFile_Maping(PFILE_MAPING p,DWORD64 _offset, DWORD64 len, char** buffer, DWORD* readed, OVERLAPPED* poverlapped)
    {
        if (p == NULL || p->mapFileH == NULL || !len)
        {
            return ReadFile(p->fileH,*buffer, len, readed, poverlapped);
        }
        //if (len > MAP_BUFFER_LEN)
        //    return 0;
        SIZE_T llen = mGetFileSize(p);
        if (p->map_buffer && _offset >= p->buffer_offset && (_offset + len) < (p->mapsize + p->buffer_offset))
        {
            //*buffer = map_buffer + offset - buffer_offset;
            memcpy(*buffer, p->map_buffer + _offset - p->buffer_offset, len);
        }
        else
        {
            if (p->map_buffer)
            {
                UnmapViewOfFile(p->map_buffer);
                p->map_buffer = NULL;
                p->buffer_offset = NULL;
                p->mapsize = NULL;
            }
            static DWORD dwAllocationGranularity = 0;
            if (!p->flag)
            {
                p->flag = 1;
                SYSTEM_INFO sinf = { 0 };
                GetSystemInfo(&sinf);
                dwAllocationGranularity = sinf.dwAllocationGranularity;
            }
            DWORD64 needmapoffset = 0;
            if(_offset)
                needmapoffset = _offset - _offset % dwAllocationGranularity;
            SIZE_T mapsize = MAP_BUFFER_LEN;
            if (len > MAP_BUFFER_LEN)
				mapsize = (SIZE_T)len;
			if (needmapoffset + mapsize > llen)
				mapsize = llen - needmapoffset;
            p->map_buffer = (char*)MapViewOfFile(p->mapFileH, FILE_MAP_READ, *(DWORD*)((char*)&needmapoffset + 4), (DWORD)(needmapoffset & 0xFFFFFFFF), mapsize);
			if (!p->map_buffer)
			{
				//return 0;
				return ReadFile(p->fileH, *buffer, len, readed, poverlapped);
			}
			if (_offset > llen)
			{
				return ReadFile(p->fileH, *buffer, len, readed, poverlapped);
			}
            p->buffer_offset = needmapoffset;
            p->mapsize = mapsize;
            //*buffer = map_buffer + offset - buffer_offset;
			memcpy(*buffer, p->map_buffer + _offset - needmapoffset, len);
        }
        *readed = len;
        return 1;
    }
    SIZE_T mGetFileSize(PFILE_MAPING p)
    {
        if (p->filesize)
            return  p->filesize;
        if (p->fileH)
        {
            *(DWORD*)&p->filesize = GetFileSize(p->fileH, (DWORD*)((char*)&p->filesize + 4));
            return  p->filesize;
        }
        return 0;
    }