Add check "Do not index booleans"

[mfvanek]

Extracted from #45

See the article by Kirill Borovikov posted on Habr https://habr.com/ru/companies/tensor/articles/488104/
Part "Bad: too little (boolean)".

1. Add new sql-query to https://github.com/mfvanek/pg-index-health-sql
2. Then implement new check with tests
3. After that add example to demo apps

[Evreke]

Here is prototype of request that select indexes with boolean type in it.

select -- pn.nspname,
       pi.indrelid::regclass::text   as table_name,
       pi.indexrelid::regclass::text as index_name,
       col.attname                   as column_name,
       col.attnotnull                as column_not_null
-- col.atttypid::regtype::text   as column_type
from pg_catalog.pg_index pi
         join pg_catalog.pg_class pc on pc.oid = pi.indexrelid
         join pg_catalog.pg_namespace pn on pn.oid = pc.relnamespace
         join pg_catalog.pg_attribute col on col.attrelid = pi.indrelid and col.attnum = any (pi.indkey)
where pn.nspname = 'public'::text
  and NOT pi.indisunique
  AND pi.indisready
  AND pi.indisvalid
  AND col.atttypid = 'boolean'::regtype
order by table_name, index_name;

[Evreke]

I'm going to implement this feature this week. Sorry for delay.

[mfvanek]

Need to

• update readme
• update spring boot starter
• add sample to the demo apps