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Require a minimum fanout for interior nodes. This prevents cases

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where excessively large terms keep the tree from finding a single
root.  A downside is that this could result in large interior nodes in
the presence of large terms, which may be prone to fragmentation,
though if the nodes were smaller that would translate into more levels
in the tree, which would also have that problem. (CVS 3510)

FossilOrigin-Name: 64b7e3406134ac4891113b9bb432ad97504268bb
This commit is contained in:
shess
2006-11-13 21:00:54 +00:00
parent 545311eeca
commit debbcdfead
3 changed files with 30 additions and 12 deletions
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28
ext/fts2/fts2.c
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@ -159,9 +159,11 @@
** iBlockid+i.
**
** New data is spilled to a new interior node at the same height when
** the current node exceeds INTERIOR_MAX bytes (default 2048). The
** interior nodes at a given height are naturally tracked by interior
** nodes at height+1, and so on.
** the current node exceeds INTERIOR_MAX bytes (default 2048).
** INTERIOR_MIN_TERMS (default 7) keeps large terms from monopolizing
** interior nodes and making the tree too skinny. The interior nodes
** at a given height are naturally tracked by interior nodes at
** height+1, and so on.
**
**
**** Segment directory ****
@ -3598,6 +3600,16 @@ static int index_update(fulltext_vtab *v, sqlite_int64 iRow,
/* How large interior nodes can grow. */
#define INTERIOR_MAX 2048
/* Minimum number of terms per interior node (except the root). This
** prevents large terms from making the tree too skinny - must be >0
** so that the tree always makes progress. Note that the min tree
** fanout will be INTERIOR_MIN_TERMS+1.
*/
#define INTERIOR_MIN_TERMS 7
#if INTERIOR_MIN_TERMS<1
# error INTERIOR_MIN_TERMS must be greater than 0.
#endif
/* ROOT_MAX controls how much data is stored inline in the segment
** directory.
*/
@ -3642,6 +3654,7 @@ typedef struct InteriorWriter {
InteriorBlock *first, *last;
struct InteriorWriter *parentWriter;
sqlite_int64 iOpeningChildBlock; /* First child block in block "last". */
#ifndef NDEBUG
sqlite_int64 iLastChildBlock; /* for consistency checks. */
#endif
@ -3659,6 +3672,7 @@ static void interiorWriterInit(int iHeight, const char *pTerm, int nTerm,
CLEAR(pWriter);
pWriter->iHeight = iHeight;
pWriter->iOpeningChildBlock = iChildBlock;
#ifndef NDEBUG
pWriter->iLastChildBlock = iChildBlock;
#endif
@ -3680,11 +3694,15 @@ static void interiorWriterAppend(InteriorWriter *pWriter,
#endif
assert( pWriter->iLastChildBlock==iChildBlock );
if( pWriter->last->data.nData+n+nTerm>INTERIOR_MAX ){
/* Overflow to a new block. */
/* Overflow to a new block if the new term makes the current block
** too big, and the current block already has enough terms.
*/
if( pWriter->last->data.nData+n+nTerm>INTERIOR_MAX &&
iChildBlock-pWriter->iOpeningChildBlock>INTERIOR_MIN_TERMS ){
pWriter->last->next = interiorBlockNew(pWriter->iHeight, iChildBlock,
pTerm, nTerm);
pWriter->last = pWriter->last->next;
pWriter->iOpeningChildBlock = iChildBlock;
}else{
dataBufferAppend2(&pWriter->last->data, c, n, pTerm, nTerm);
}