On Mon, Jul 11, 2022 at 03:21:38PM +1200, Nathan Ward wrote:
> > Note that postgres doesn't automatically analyze parent tables, so you should
> > maybe do that whenever the data changes enough for it to matter.
>
> Hmm. This raises some stuff I’m not familiar with - does analysing a parent table do anything?
Yes
You could check if you have stats now (maybe due to a global ANALYZE or
analyzedb) and how the query plans change if you analyze.
The transaction may be overly conservative.
SELECT COUNT(1) FROM pg_stats WHERE tablename=PARENT;
SELECT last_analyze, last_autoanalyze, relname FROM pg_stat_all_tables WHERE relname=PARENT;
begin;
SET default_statistics_target=10;
ANALYZE;
explain SELECT [...];
rollback;
> I got the impression that analysing the parent was just shorthand for analysing all of the attached partitions.
Could you let us know if the documentation left that impression ?
See here (this was updated recently).
https://www.postgresql.org/docs/13/sql-analyze.html#id-1.9.3.46.8
For partitioned tables, ANALYZE gathers statistics by sampling rows from all partitions; in addition, it will recurse
intoeach partition and update its statistics. Each leaf partition is analyzed only once, even with multi-level
partitioning.No statistics are collected for only the parent table (without data from its partitions), because with
partitioningit's guaranteed to be empty.
By contrast, if the table being analyzed has inheritance children, ANALYZE gathers two sets of statistics: one on the
rowsof the parent table only, and a second including rows of both the parent table and all of its children. This second
setof statistics is needed when planning queries that process the inheritance tree as a whole. The child tables
themselvesare not individually analyzed in this case.
The autovacuum daemon does not process partitioned tables, nor does it process inheritance parents if only the children
areever modified. It is usually necessary to periodically run a manual ANALYZE to keep the statistics of the table
hierarchyup to date.
> Perhaps because I attach a table with data, the parent sometimes decides it needs to run analyse on a bunch of
things?
No, that doesn't happen.
> Or, maybe it uses the most recently attached partition, with bad statistics, to plan queries that only touch other
partitions?
This is closer to what I was talking about.
To be clear, you are using relkind=p partitions (added in v10), and not
inheritance parents, right ?
--
Justin
