Request: Add to config values from parent config

Is your feature request related to a problem? Please describe.

Can use source/index inheritance in the config files to cut down on duplication etc. But when (re)define a multi-value config variable (like sql_query_pre, or sql_attr_multi) in the child config, it completely overrides the values defined in the parent.

This is particularly common with sql_query_pre, where a a 'template' index can define variables like 'query_cache_type' and 'max_statement_time' etc.

... but means when a child index needs to add a sql_query_pre, (eg to setup a counter in a main+delta setup), then needs to duplicate all the queries from the template index as well.

Describe the solution you'd like

Most universal would be a way explicitly add to existing, rather than starting again, Perhaps something like

sql_query_pre           += REPLACE INTO sph_counter SELECT .... 

(ie += to explicitly to add to value(s) from the parent, rather than starting again in the child)

Describe alternatives you've considered

In the case of main+delta, could perhaps move the counter update into a _post, particully using the $maxid macro. (rather than adding to _pre). But its not a universal solution, as some _pre do other stuff, like creating a temporary table, or setting up locks needed for some specific indexes.

Another perhaps simpler idea would be TWO sets of sql_query_pre queries. E.g. have both sql_query_setup and sql_query_pre, both simply run querie(s) before main index query. Means sql_query_setup (for example) could be defined in the template, and sql_query_pre would be in the child indexes.

... in fact this 'sql_query_setup' might be useful, as it could ALSO be run when reconnect to run the 'sql_attr_multi' queries. Ie sql_query_setup is run for ALL mysql connections, but sql_query_pre is only run on the 'main' index connection.

Additional context

This is not a particularly important request, as it all it would do is simplify config files, but it could really help on complex setups.

https://github.com/geograph-project/geograph-project/blob/british-isles/system/docker/manticore/etc/sphinxsearch/sphinx.conf.d/sample8E.conf
is an example index, that needs to redefine all the sql_query_pre, which already defined in the parent index.
https://github.com/geograph-project/geograph-project/blob/british-isles/system/docker/manticore/etc/sphinxsearch/sphinx.conf.d/21-database.conf

Start by reviewing the inheritance and repeated sql_query_pre definitions in sample8E.conf and 21-database.conf linked in the issue. Compare the proposed += behavior with the sql_query_setup alternative; the work is done when a child configuration can add to parent multi-value settings without duplicating them and the resulting query execution behavior is covered.