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When computing STAT1 values using ANALYZE, if a ratio comes out to be between

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1.0 and 1.1, then round it down to 1 rather than the using the default rounding
rule of changing it to 2.  The reduces the estimation error for the
case where a column value is very nearly, but not quite unique.

FossilOrigin-Name: eb59c46a5aed69bc6fd096997bf24c082e533c1085439f6ec1fbe5ff78e8b374
This commit is contained in:
drh
2022-04-27 16:41:56 +00:00
parent 9684d71081
commit 609959285b
3 changed files with 15 additions and 9 deletions
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8
src/analyze.c
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@@ -847,9 +847,14 @@ static void statGet(
** * "WHERE a=? AND b=?" matches 2 rows.
**
** If D is the count of distinct values and K is the total number of
** rows, then each estimate is computed as:
** rows, then each estimate is usually computed as:
**
** I = (K+D-1)/D
**
** In other words, I is K/D rounded up to the next whole integer.
** However, if I is between 1.0 and 1.1 (in other words if I is
** close to 1.0 but just a little larger) then do not round up but
** instead keep the I value at 1.0.
*/
sqlite3_str sStat; /* Text of the constructed "stat" line */
int i; /* Loop counter */
@@ -860,6 +865,7 @@ static void statGet(
for(i=0; i<p->nKeyCol; i++){
u64 nDistinct = p->current.anDLt[i] + 1;
u64 iVal = (p->nRow + nDistinct - 1) / nDistinct;
if( iVal==2 && p->nRow*10 <= nDistinct*11 ) iVal = 1;
sqlite3_str_appendf(&sStat, " %llu", iVal);
assert( p->current.anEq[i] );
}