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Fix compute_scalar_stats() for case that all values exceed WIDTH_THRESHOLD.

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The standard typanalyze functions skip over values whose detoasted size
exceeds WIDTH_THRESHOLD (1024 bytes), so as to limit memory bloat during
ANALYZE.  However, we (I think I, actually :-() failed to consider the
possibility that *every* non-null value in a column is too wide.  While
compute_minimal_stats() seems to behave reasonably anyway in such a case,
compute_scalar_stats() just fell through and generated no pg_statistic
entry at all.  That's unnecessarily pessimistic: we can still produce
valid stanullfrac and stawidth values in such cases, since we do include
too-wide values in the average-width calculation.  Furthermore, since the
general assumption in this code is that too-wide values are probably all
distinct from each other, it seems reasonable to set stadistinct to -1
("all distinct").

Per complaint from Kadri Raudsepp.  This has been like this since roughly
neolithic times, so back-patch to all supported branches.
This commit is contained in:
Tom Lane
2014-01-11 13:41:41 -05:00
parent fd2ace8028
commit 6286526207
1 changed files with 15 additions and 1 deletions
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16
src/backend/commands/analyze.c
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@@ -2732,7 +2732,21 @@ compute_scalar_stats(VacAttrStatsP stats,
slot_idx++;
}
}
else if (nonnull_cnt == 0 && null_cnt > 0)
else if (nonnull_cnt > 0)
{
/* We found some non-null values, but they were all too wide */
Assert(nonnull_cnt == toowide_cnt);
stats->stats_valid = true;
/* Do the simple null-frac and width stats */
stats->stanullfrac = (double) null_cnt / (double) samplerows;
if (is_varwidth)
stats->stawidth = total_width / (double) nonnull_cnt;
else
stats->stawidth = stats->attrtype->typlen;
/* Assume all too-wide values are distinct, so it's a unique column */
stats->stadistinct = -1.0;
}
else if (null_cnt > 0)
{
/* We found only nulls; assume the column is entirely null */
stats->stats_valid = true;