Improve admin dashboard summary display to prevent timeout

[wxiaoguang]

Description

The Admin -> Dashboard -> Summary shows many counting numbers. Some of them aren't useful to admins. Even worse, some counting operations on the large tables like action will make the page timeout. For example:

• 504 on /admin after Gitea upgrade because of large action table #17504 , "Action table contains 8539538 rows", it takes several minutes to count.

I suggest to improve the summary information, we shouldn't count so many models. Each number displayed here should be useful to admins.

[zeripath]

Instead of these COUNT(*) calls I think we need to use something like:

SELECT table_rows FROM information_schema.tables WHERE tables.table_name = ? AND tables.table_schema = ?;

with the first parameter as the table name and the second param is the db name.

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Of course table_rows is a MySQL extension.

Postgres needs:

SELECT reltuples AS estimate FROM pg_class WHERE relname = 'table_name';

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SQLite it appears that COUNT is just the fastest available.

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MSSQL

sp_spaceused 'Table_name`

[zeripath]

I have to say I suspect that getting the count for the Action table might be a false report - the real problems are if you have:

[metrics]
ENABLED_ISSUE_BY_LABEL=true 
ENABLED_ISSUE_BY_REPOSITORY=true

Then all of that work occurs every time you look at the admin dashboard page.

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Thus we have to cache these statistics or remove these from the GetStatistics call.

[wxiaoguang]

For the user (#17504), it's not a false report, after the user truncated their action table, then their system functions correctly.

For me, it's not a false report. Now what I am doing is truncating my action table regularly. (I know how to tune MySQL to bypass the problem, but the truncating just reminds me that there is still a problem).

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ps: other statistics also have performance problem, too, so caching them and moving them to a dedicated page are all good.