Re: why does reindex invalidate relcache without modifying system tables

> 2021年12月27日 23:54，Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> 写道：
>
> wenjing zeng <wjzeng2012(at)gmail(dot)com> writes:
>> I found that in the index_update_stats function, i.e. the CREATE INDEX/REINDEX/Truncate INDEX process,
>> relchche is invalidated whether the index information is updated. I want to know why you're did this
>
> Did you read the function's header comment? It says
>
> * NOTE: an important side-effect of this operation is that an SI invalidation
> * message is sent out to all backends --- including me --- causing relcache
> * entries to be flushed or updated with the new data. This must happen even
> * if we find that no change is needed in the pg_class row. When updating
> * a heap entry, this ensures that other backends find out about the new
> * index. When updating an index, it's important because some index AMs
> * expect a relcache flush to occur after REINDEX.
>
> That is, what we need to force an update of is either the relcache's
> rd_indexlist list (for a table) or rd_amcache (for an index).
>
> In the REINDEX case, we could conceivably skip the flush on the table,
> but not on the index. I don't think it's worth worrying about though,
> because REINDEX will very probably have an update for the table's
> physical size data (relpages and/or reltuples), so that it's unlikely
> that the no-change path would be taken anyway.
>
> regards, tom lane
Thank you for your explanation, which clears up my doubts.

Wenjing