"Tom Lane" <[EMAIL PROTECTED]> writes: > > Also, I would like to see the actual test code. I wonder whether what > you measured is the ability of the compiler to optimize references to > successive elements of an array inside a loop; that has little or > nothing to do with the typical usage of BufferGetBlock(). >
The source code is attached. compiled with "gcc testbuf.c". I tried -O2 actually, and it turns out that the timing is reduced a lot so not believable. --- /* * testbuf.c */ #include <stdio.h> #include <sys/file.h> #include <sys/param.h> #include <sys/stat.h> #include <sys/time.h> #include <unistd.h> #include <fcntl.h> #define BLCKSZ 8192 #define NBuffers 80000 typedef void* Block; int main(void) { int i, round, method; Block k, start; struct timeval start_t, stop_t; long usecs; Block *array = (Block *) calloc(NBuffers, sizeof(Block)); start = (Block)0xff3386; for (i = 0; i < NBuffers; i++) array[i] = start + BLCKSZ*i; for (method = 0; method < 3; method ++) { start = (Block)0xff3386; for (round = 0; round < 3; round ++) { gettimeofday(&start_t, NULL); if (method == 0) { for (i = 0; i < NBuffers; i++) k = array[i]; } if (method == 1) { for (i = 0; i < NBuffers; i++) k = start + i*BLCKSZ; } if (method == 2) { for (i = 0; i < NBuffers; i++) k = start + (i<<13); } gettimeofday(&stop_t, NULL); if (stop_t.tv_usec < start_t.tv_usec) { stop_t.tv_sec--; stop_t.tv_usec += 1000000; } usecs = (long) (stop_t.tv_sec - start_t.tv_sec) * 1000000 + (long) (stop_t.tv_usec - start_t.tv_usec); fprintf (stdout, "duration round %d of %s method: %ld.%03ld ms\n", round + 1, method==0?"array":method==1?"mul":"shift", (long) ((stop_t.tv_sec - start_t.tv_sec) * 1000 + (stop_t.tv_usec - start_t.tv_usec) / 1000), (long) (stop_t.tv_usec - start_t.tv_usec) % 1000); } } } ---------------------------(end of broadcast)--------------------------- TIP 9: In versions below 8.0, the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match