granicus.if.org Git - postgresql/blob - src/backend/access/hash/hashsearch.c

   1 /*-------------------------------------------------------------------------
   2  *
   3  * hashsearch.c
   4  *        search code for postgres hash tables
   5  *
   6  * Portions Copyright (c) 1996-2006, PostgreSQL Global Development Group
   7  * Portions Copyright (c) 1994, Regents of the University of California
   8  *
   9  *
  10  * IDENTIFICATION
  11  *        $PostgreSQL: pgsql/src/backend/access/hash/hashsearch.c,v 1.45 2006/07/14 14:52:17 momjian Exp $
  12  *
  13  *-------------------------------------------------------------------------
  14  */
  15 #include "postgres.h"
  16
  17 #include "access/hash.h"
  18 #include "pgstat.h"
  19
  20
  21 /*
  22  *      _hash_next() -- Get the next item in a scan.
  23  *
  24  *              On entry, we have a valid currentItemData in the scan, and a
  25  *              pin and read lock on the page that contains that item.
  26  *              We find the next item in the scan, if any.
  27  *              On success exit, we have the page containing the next item
  28  *              pinned and locked.
  29  */
  30 bool
  31 _hash_next(IndexScanDesc scan, ScanDirection dir)
  32 {
  33         Relation        rel = scan->indexRelation;
  34         HashScanOpaque so = (HashScanOpaque) scan->opaque;
  35         Buffer          buf;
  36         Page            page;
  37         OffsetNumber offnum;
  38         ItemPointer current;
  39         IndexTuple      itup;
  40
  41         /* we still have the buffer pinned and read-locked */
  42         buf = so->hashso_curbuf;
  43         Assert(BufferIsValid(buf));
  44
  45         /*
  46          * step to next valid tuple.
  47          */
  48         if (!_hash_step(scan, &buf, dir))
  49                 return false;
  50
  51         /* if we're here, _hash_step found a valid tuple */
  52         current = &(scan->currentItemData);
  53         offnum = ItemPointerGetOffsetNumber(current);
  54         _hash_checkpage(rel, buf, LH_BUCKET_PAGE | LH_OVERFLOW_PAGE);
  55         page = BufferGetPage(buf);
  56         itup = (IndexTuple) PageGetItem(page, PageGetItemId(page, offnum));
  57         scan->xs_ctup.t_self = itup->t_tid;
  58
  59         return true;
  60 }
  61
  62 /*
  63  * Advance to next page in a bucket, if any.
  64  */
  65 static void
  66 _hash_readnext(Relation rel,
  67                            Buffer *bufp, Page *pagep, HashPageOpaque *opaquep)
  68 {
  69         BlockNumber blkno;
  70
  71         blkno = (*opaquep)->hasho_nextblkno;
  72         _hash_relbuf(rel, *bufp);
  73         *bufp = InvalidBuffer;
  74         if (BlockNumberIsValid(blkno))
  75         {
  76                 *bufp = _hash_getbuf(rel, blkno, HASH_READ);
  77                 _hash_checkpage(rel, *bufp, LH_OVERFLOW_PAGE);
  78                 *pagep = BufferGetPage(*bufp);
  79                 *opaquep = (HashPageOpaque) PageGetSpecialPointer(*pagep);
  80         }
  81 }
  82
  83 /*
  84  * Advance to previous page in a bucket, if any.
  85  */
  86 static void
  87 _hash_readprev(Relation rel,
  88                            Buffer *bufp, Page *pagep, HashPageOpaque *opaquep)
  89 {
  90         BlockNumber blkno;
  91
  92         blkno = (*opaquep)->hasho_prevblkno;
  93         _hash_relbuf(rel, *bufp);
  94         *bufp = InvalidBuffer;
  95         if (BlockNumberIsValid(blkno))
  96         {
  97                 *bufp = _hash_getbuf(rel, blkno, HASH_READ);
  98                 _hash_checkpage(rel, *bufp, LH_BUCKET_PAGE | LH_OVERFLOW_PAGE);
  99                 *pagep = BufferGetPage(*bufp);
 100                 *opaquep = (HashPageOpaque) PageGetSpecialPointer(*pagep);
 101         }
 102 }
 103
 104 /*
 105  *      _hash_first() -- Find the first item in a scan.
 106  *
 107  *              Find the first item in the index that
 108  *              satisfies the qualification associated with the scan descriptor. On
 109  *              success, the page containing the current index tuple is read locked
 110  *              and pinned, and the scan's opaque data entry is updated to
 111  *              include the buffer.
 112  */
 113 bool
 114 _hash_first(IndexScanDesc scan, ScanDirection dir)
 115 {
 116         Relation        rel = scan->indexRelation;
 117         HashScanOpaque so = (HashScanOpaque) scan->opaque;
 118         uint32          hashkey;
 119         Bucket          bucket;
 120         BlockNumber blkno;
 121         Buffer          buf;
 122         Buffer          metabuf;
 123         Page            page;
 124         HashPageOpaque opaque;
 125         HashMetaPage metap;
 126         IndexTuple      itup;
 127         ItemPointer current;
 128         OffsetNumber offnum;
 129
 130         pgstat_count_index_scan(&scan->xs_pgstat_info);
 131
 132         current = &(scan->currentItemData);
 133         ItemPointerSetInvalid(current);
 134
 135         /*
 136          * We do not support hash scans with no index qualification, because we
 137          * would have to read the whole index rather than just one bucket. That
 138          * creates a whole raft of problems, since we haven't got a practical way
 139          * to lock all the buckets against splits or compactions.
 140          */
 141         if (scan->numberOfKeys < 1)
 142                 ereport(ERROR,
 143                                 (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
 144                                  errmsg("hash indexes do not support whole-index scans")));
 145
 146         /*
 147          * If the constant in the index qual is NULL, assume it cannot match any
 148          * items in the index.
 149          */
 150         if (scan->keyData[0].sk_flags & SK_ISNULL)
 151                 return false;
 152
 153         /*
 154          * Okay to compute the hash key.  We want to do this before acquiring any
 155          * locks, in case a user-defined hash function happens to be slow.
 156          */
 157         hashkey = _hash_datum2hashkey(rel, scan->keyData[0].sk_argument);
 158
 159         /*
 160          * Acquire shared split lock so we can compute the target bucket safely
 161          * (see README).
 162          */
 163         _hash_getlock(rel, 0, HASH_SHARE);
 164
 165         /* Read the metapage */
 166         metabuf = _hash_getbuf(rel, HASH_METAPAGE, HASH_READ);
 167         _hash_checkpage(rel, metabuf, LH_META_PAGE);
 168         metap = (HashMetaPage) BufferGetPage(metabuf);
 169
 170         /*
 171          * Compute the target bucket number, and convert to block number.
 172          */
 173         bucket = _hash_hashkey2bucket(hashkey,
 174                                                                   metap->hashm_maxbucket,
 175                                                                   metap->hashm_highmask,
 176                                                                   metap->hashm_lowmask);
 177
 178         blkno = BUCKET_TO_BLKNO(metap, bucket);
 179
 180         /* done with the metapage */
 181         _hash_relbuf(rel, metabuf);
 182
 183         /*
 184          * Acquire share lock on target bucket; then we can release split lock.
 185          */
 186         _hash_getlock(rel, blkno, HASH_SHARE);
 187
 188         _hash_droplock(rel, 0, HASH_SHARE);
 189
 190         /* Update scan opaque state to show we have lock on the bucket */
 191         so->hashso_bucket = bucket;
 192         so->hashso_bucket_valid = true;
 193         so->hashso_bucket_blkno = blkno;
 194
 195         /* Fetch the primary bucket page for the bucket */
 196         buf = _hash_getbuf(rel, blkno, HASH_READ);
 197         _hash_checkpage(rel, buf, LH_BUCKET_PAGE);
 198         page = BufferGetPage(buf);
 199         opaque = (HashPageOpaque) PageGetSpecialPointer(page);
 200         Assert(opaque->hasho_bucket == bucket);
 201
 202         /* If a backwards scan is requested, move to the end of the chain */
 203         if (ScanDirectionIsBackward(dir))
 204         {
 205                 while (BlockNumberIsValid(opaque->hasho_nextblkno))
 206                         _hash_readnext(rel, &buf, &page, &opaque);
 207         }
 208
 209         /* Now find the first tuple satisfying the qualification */
 210         if (!_hash_step(scan, &buf, dir))
 211                 return false;
 212
 213         /* if we're here, _hash_step found a valid tuple */
 214         offnum = ItemPointerGetOffsetNumber(current);
 215         _hash_checkpage(rel, buf, LH_BUCKET_PAGE | LH_OVERFLOW_PAGE);
 216         page = BufferGetPage(buf);
 217         itup = (IndexTuple) PageGetItem(page, PageGetItemId(page, offnum));
 218         scan->xs_ctup.t_self = itup->t_tid;
 219
 220         return true;
 221 }
 222
 223 /*
 224  *      _hash_step() -- step to the next valid item in a scan in the bucket.
 225  *
 226  *              If no valid record exists in the requested direction, return
 227  *              false.  Else, return true and set the CurrentItemData for the
 228  *              scan to the right thing.
 229  *
 230  *              'bufP' points to the current buffer, which is pinned and read-locked.
 231  *              On success exit, we have pin and read-lock on whichever page
 232  *              contains the right item; on failure, we have released all buffers.
 233  */
 234 bool
 235 _hash_step(IndexScanDesc scan, Buffer *bufP, ScanDirection dir)
 236 {
 237         Relation        rel = scan->indexRelation;
 238         HashScanOpaque so = (HashScanOpaque) scan->opaque;
 239         ItemPointer current;
 240         Buffer          buf;
 241         Page            page;
 242         HashPageOpaque opaque;
 243         OffsetNumber maxoff;
 244         OffsetNumber offnum;
 245         BlockNumber blkno;
 246         IndexTuple      itup;
 247
 248         current = &(scan->currentItemData);
 249
 250         buf = *bufP;
 251         _hash_checkpage(rel, buf, LH_BUCKET_PAGE | LH_OVERFLOW_PAGE);
 252         page = BufferGetPage(buf);
 253         opaque = (HashPageOpaque) PageGetSpecialPointer(page);
 254
 255         /*
 256          * If _hash_step is called from _hash_first, current will not be valid, so
 257          * we can't dereference it.  However, in that case, we presumably want to
 258          * start at the beginning/end of the page...
 259          */
 260         maxoff = PageGetMaxOffsetNumber(page);
 261         if (ItemPointerIsValid(current))
 262                 offnum = ItemPointerGetOffsetNumber(current);
 263         else
 264                 offnum = InvalidOffsetNumber;
 265
 266         /*
 267          * 'offnum' now points to the last tuple we have seen (if any).
 268          *
 269          * continue to step through tuples until: 1) we get to the end of the
 270          * bucket chain or 2) we find a valid tuple.
 271          */
 272         do
 273         {
 274                 switch (dir)
 275                 {
 276                         case ForwardScanDirection:
 277                                 if (offnum != InvalidOffsetNumber)
 278                                         offnum = OffsetNumberNext(offnum);      /* move forward */
 279                                 else
 280                                         offnum = FirstOffsetNumber; /* new page */
 281
 282                                 while (offnum > maxoff)
 283                                 {
 284                                         /*
 285                                          * either this page is empty (maxoff ==
 286                                          * InvalidOffsetNumber) or we ran off the end.
 287                                          */
 288                                         _hash_readnext(rel, &buf, &page, &opaque);
 289                                         if (BufferIsValid(buf))
 290                                         {
 291                                                 maxoff = PageGetMaxOffsetNumber(page);
 292                                                 offnum = FirstOffsetNumber;
 293                                         }
 294                                         else
 295                                         {
 296                                                 /* end of bucket */
 297                                                 maxoff = offnum = InvalidOffsetNumber;
 298                                                 break;  /* exit while */
 299                                         }
 300                                 }
 301                                 break;
 302
 303                         case BackwardScanDirection:
 304                                 if (offnum != InvalidOffsetNumber)
 305                                         offnum = OffsetNumberPrev(offnum);      /* move back */
 306                                 else
 307                                         offnum = maxoff;        /* new page */
 308
 309                                 while (offnum < FirstOffsetNumber)
 310                                 {
 311                                         /*
 312                                          * either this page is empty (offnum ==
 313                                          * InvalidOffsetNumber) or we ran off the end.
 314                                          */
 315                                         _hash_readprev(rel, &buf, &page, &opaque);
 316                                         if (BufferIsValid(buf))
 317                                                 maxoff = offnum = PageGetMaxOffsetNumber(page);
 318                                         else
 319                                         {
 320                                                 /* end of bucket */
 321                                                 maxoff = offnum = InvalidOffsetNumber;
 322                                                 break;  /* exit while */
 323                                         }
 324                                 }
 325                                 break;
 326
 327                         default:
 328                                 /* NoMovementScanDirection */
 329                                 /* this should not be reached */
 330                                 break;
 331                 }
 332
 333                 /* we ran off the end of the world without finding a match */
 334                 if (offnum == InvalidOffsetNumber)
 335                 {
 336                         *bufP = so->hashso_curbuf = InvalidBuffer;
 337                         ItemPointerSetInvalid(current);
 338                         return false;
 339                 }
 340
 341                 /* get ready to check this tuple */
 342                 itup = (IndexTuple) PageGetItem(page, PageGetItemId(page, offnum));
 343         } while (!_hash_checkqual(scan, itup));
 344
 345         /* if we made it to here, we've found a valid tuple */
 346         blkno = BufferGetBlockNumber(buf);
 347         *bufP = so->hashso_curbuf = buf;
 348         ItemPointerSet(current, blkno, offnum);
 349         return true;
 350 }