Dear MySQL experts:
Thank you so much for your reply to my previous Qs, they are very
helpful!
Could someone please help me understand function my_hash_insert()
in mysys/hash.cc?
what are lines 352 -429 trying to achieve? Are they just some
optimization to shuffle existing
hash entries in the table (since the existing hash entries may be in
the wrong slot due to chaining
in the case of hash collision)?
Thanks!
yan
-----------------------
I copied function my_hash_insert() below:
331 my_bool my_hash_insert(HASH *info, const uchar *record)
332 {
333 int flag;
334 size_t idx,halfbuff,hash_nr,first_index;
335 uchar *UNINIT_VAR(ptr_to_rec),*UNINIT_VAR(ptr_to_rec2);
336 HASH_LINK *data,*empty,*UNINIT_VAR(gpos),*UNINIT_VAR(gpos2),*pos;
337
338 if (HASH_UNIQUE & info->flags)
339 {
340 uchar *key= (uchar*) my_hash_key(info, record, &idx, 1);
341 if (my_hash_search(info, key, idx))
342 return(TRUE); /* Duplicate entry */
343 }
344
345 flag=0;
346 if (!(empty=(HASH_LINK*) alloc_dynamic(&info->array)))
347 return(TRUE); /* No more memory */
348
349 data=dynamic_element(&info->array,0,HASH_LINK*);
350 halfbuff= info->blength >> 1;
351
352 idx=first_index=info->records-halfbuff;
353 if (idx != info->records) /* If some records */
354 {
355 do
356 {
357 pos=data+idx;
358 hash_nr=rec_hashnr(info,pos->data);
359 if (flag == 0) /* First loop; Check if ok */
360 if (my_hash_mask(hash_nr, info->blength, info->records) !=
first_index)
361 break;
362 if (!(hash_nr & halfbuff))
363 { /* Key will not move */
364 if (!(flag & LOWFIND))
365 {
366 if (flag & HIGHFIND)
367 {
368 flag=LOWFIND | HIGHFIND;
369 /* key shall be moved to the current empty position */
370 gpos=empty;
371 ptr_to_rec=pos->data;
372 empty=pos; /* This place is now free */
373 }
374 else
375 {
376 flag=LOWFIND | LOWUSED; /* key isn't changed */
377 gpos=pos;
378 ptr_to_rec=pos->data;
379 }
380 }
381 else
382 {
383 if (!(flag & LOWUSED))
384 {
385 /* Change link of previous LOW-key */
386 gpos->data=ptr_to_rec;
387 gpos->next= (uint) (pos-data);
388 flag= (flag & HIGHFIND) | (LOWFIND | LOWUSED);
389 }
390 gpos=pos;
391 ptr_to_rec=pos->data;
392 }
393 }
394 else
395 { /* key will be moved */
396 if (!(flag & HIGHFIND))
397 {
398 flag= (flag & LOWFIND) | HIGHFIND;
399 /* key shall be moved to the last (empty) position */
400 gpos2 = empty; empty=pos;
401 ptr_to_rec2=pos->data;
402 }
403 else
404 {
405 if (!(flag & HIGHUSED))
406 {
407 /* Change link of previous hash-key and save */
408 gpos2->data=ptr_to_rec2;
409 gpos2->next=(uint) (pos-data);
410 flag= (flag & LOWFIND) | (HIGHFIND | HIGHUSED);
411 }
412 gpos2=pos;
413 ptr_to_rec2=pos->data;
414 }
415 }
416 }
417 while ((idx=pos->next) != NO_RECORD);
418
419 if ((flag & (LOWFIND | LOWUSED)) == LOWFIND)
420 {
421 gpos->data=ptr_to_rec;
422 gpos->next=NO_RECORD;
423 }
424 if ((flag & (HIGHFIND | HIGHUSED)) == HIGHFIND)
425 {
426 gpos2->data=ptr_to_rec2;
427 gpos2->next=NO_RECORD;
428 }
429 }
430 /* Check if we are at the empty position */
431
432 idx= my_hash_mask(rec_hashnr(info, record), info->blength, info-
>records + 1);
433 pos=data+idx;
434 if (pos == empty)
435 {
436 pos->data=(uchar*) record;
437 pos->next=NO_RECORD;
438 }
439 else
440 {
441 /* Check if more records in same hash-nr family */
442 empty[0]=pos[0];
443 gpos= data + my_hash_rec_mask(info, pos, info->blength, info-
>records + 1);
444 if (pos == gpos)
445 {
446 pos->data=(uchar*) record;
447 pos->next=(uint) (empty - data);
448 }
449 else
450 {
451
452 pos->data=(uchar*) record;
453 pos->next=NO_RECORD;
457 movelink(data,(uint) (pos-data),(uint) (gpos-data),(uint)
(empty-data));
458 }
459 }
460 if (++info->records == info->blength)
461 info->blength+= info->blength;
462 return(0);
463 }
464
