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Commit36785a2

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Fix compute_scalar_stats() for case that all values exceed WIDTH_THRESHOLD.
The standard typanalyze functions skip over values whose detoasted sizeexceeds WIDTH_THRESHOLD (1024 bytes), so as to limit memory bloat duringANALYZE. However, we (I think I, actually :-() failed to consider thepossibility that *every* non-null value in a column is too wide. Whilecompute_minimal_stats() seems to behave reasonably anyway in such a case,compute_scalar_stats() just fell through and generated no pg_statisticentry at all. That's unnecessarily pessimistic: we can still producevalid stanullfrac and stawidth values in such cases, since we do includetoo-wide values in the average-width calculation. Furthermore, since thegeneral assumption in this code is that too-wide values are probably alldistinct from each other, it seems reasonable to set stadistinct to -1("all distinct").Per complaint from Kadri Raudsepp. This has been like this since roughlyneolithic times, so back-patch to all supported branches.
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‎src/backend/commands/analyze.c

Lines changed: 15 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -2731,7 +2731,21 @@ compute_scalar_stats(VacAttrStatsP stats,
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slot_idx++;
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}
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}
2734-
elseif (nonnull_cnt==0&&null_cnt>0)
2734+
elseif (nonnull_cnt>0)
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{
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/* We found some non-null values, but they were all too wide */
2737+
Assert(nonnull_cnt==toowide_cnt);
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stats->stats_valid= true;
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/* Do the simple null-frac and width stats */
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stats->stanullfrac= (double)null_cnt / (double)samplerows;
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if (is_varwidth)
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stats->stawidth=total_width / (double)nonnull_cnt;
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else
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stats->stawidth=stats->attrtype->typlen;
2745+
/* Assume all too-wide values are distinct, so it's a unique column */
2746+
stats->stadistinct=-1.0;
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}
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elseif (null_cnt>0)
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{
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/* We found only nulls; assume the column is entirely null */
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stats->stats_valid= true;

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