| From: | Andres Freund <andres(at)anarazel(dot)de> |
|---|---|
| To: | legrand legrand <legrand_legrand(at)hotmail(dot)com> |
| Cc: | pgsql-performance(at)postgresql(dot)org, pgsql-hackers(at)postgresql(dot)org |
| Subject: | Re: Explain is slow with tables having many columns |
| Date: | 2018-09-24 19:50:48 |
| Message-ID: | 20180924195048.7dhnjvq5ggwjtiwi@alap3.anarazel.de |
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| Lists: | pgsql-hackers pgsql-performance |
Hi,
(CCing -hackers)
On 2018-09-24 12:22:28 -0700, legrand legrand wrote:
> I have found that explain on tables with many (hundreds) columns
> are slow compare to nominal executions.
Yea, colname_is_unique() (called via make_colname_unique()) is
essentially O(#total_columns) and rougly called once for each column in
a select list (or using or ...). IIRC we've hit this once when I was at
citus, too.
We really should be usign a more appropriate datastructure here - very
likely a hashtable. Unfortunately such a change would likely be a bit
too much to backpatch...
Greetings,
Andres Freund
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