reindex by bloat percent concurrently for postgres < 12

reindex_by_bloat_percent.sql

create extension dblink;
drop function if exists reindex_by_bloat_percent;

create or replace function reindex_by_bloat_percent (connection_string text, schema_to_process text = 'public', threshold integer = 40, min_index_bytes integer = 20971520) returns text
language PLPGSQL as $$
declare
	processed text[];
	index_info record;
	index_indexes record;
	constraint_type text;
	create_index_query text; 
	alter_table_query text;
	process_index_start timestamp;
	err_context text;
begin
	BEGIN
		perform dblink_connect('db', connection_string);

		for index_info in (select * from dblink(
			'db',
			format(
				'select indexname as index, (pg_relation_size(quote_ident(indexrelname)::text) / (case when (pg_relation_size(quote_ident(t.tablename)::text) / 100) != 0 then (pg_relation_size(quote_ident(t.tablename)::text) / 100) else 1 end)) as size_ratio_percent
				FROM pg_tables t LEFT OUTER JOIN pg_class c ON t.tablename=c.relname LEFT OUTER JOIN (SELECT c.relname AS ctablename, ipg.relname AS indexname, x.indnatts AS number_of_columns, idx_scan, idx_tup_read, idx_tup_fetch, indexrelname, indisunique FROM pg_index x JOIN pg_class c ON c.oid = x.indrelid JOIN pg_class ipg ON ipg.oid = x.indexrelid JOIN pg_stat_all_indexes psai ON x.indexrelid = psai.indexrelid ) AS foo ON t.tablename = foo.ctablename
				WHERE t.schemaname = ''%1$s'' and pg_relation_size(quote_ident(indexrelname)::text) > %3$s and ((pg_relation_size(quote_ident(t.tablename)::text) * (%2$s / 100.0)) <= pg_relation_size(quote_ident(indexrelname)::text)) ORDER BY 2 desc nulls LAST, 1',
				schema_to_process, threshold, min_index_bytes
			)
		) as t(index text, size_ratio_percent numeric))
		loop
			process_index_start := clock_timestamp();
			raise notice 'Starting to process %', index_info.index::text;
			select into constraint_type contype from dblink('db', format('select contype::text from pg_constraint where conname = ''%1$s''', index_info.index)) as t(contype text);
			select into index_indexes * from dblink(
				'db',
				format('select schemaname, tablename, indexname, tablespace, indexdef from pg_catalog.pg_indexes where indexname = ''%1$s''', index_info.index)
			) as t(schemaname text, tablename text, indexname text, tablespace text, indexdef text);

			if constraint_type = 'f' then
				raise notice '- Foreign key, ignoring (TODO?)';
				continue;
			end if;

			create_index_query := index_indexes.indexdef;
			create_index_query := replace(create_index_query, ' INDEX ' || index_info.index::text, ' INDEX CONCURRENTLY ' || index_info.index::text);
			create_index_query := replace(create_index_query, ' ' || index_info.index::text || ' ', ' ' || index_info.index::text || '_t ');

			raise notice '- Create index concurrently';
			raise notice '-- Query to execute: %', create_index_query;

			perform dblink('db', create_index_query);

			raise notice '-- New index was created, took: %', TO_CHAR(clock_timestamp() - process_index_start, 'HH24:MI:SS');

			if constraint_type = 'p' then
				raise notice '- Index % is primary key, altering', index_info.index::text;
				alter_table_query := format(
					'ALTER TABLE %1$s.%2$s DROP CONSTRAINT %3$s, ADD CONSTRAINT %3$s PRIMARY KEY USING INDEX %3$s_t',
					schema_to_process, index_indexes.tablename, index_info.index
				);
			end if;

			if constraint_type = 'u' then
				raise notice '- Index % is unique key', index_info.index::text;
				alter_table_query := format(
					'ALTER TABLE %1$s.%2$s DROP CONSTRAINT %3$s, ADD CONSTRAINT %3$s UNIQUE USING INDEX %3$s_t',
					schema_to_process, index_indexes.tablename, index_info.index
				);
			end if;

			if constraint_type is null then
				raise notice '- Index % has no constraint, drop & rename', index_info.index::text;
				alter_table_query := format(
					'DROP INDEX %3$s; ALTER INDEX %3$s_t RENAME TO %3$s;',
					schema_to_process, index_indexes.tablename, index_info.index
				);
			end if;

			raise notice '-- Query to execute: %', alter_table_query;

			perform dblink('db', alter_table_query);

			processed := array_append(processed, index_info.index::text);
			raise notice 'Reindex for % done in %', index_info.index::text, TO_CHAR(clock_timestamp() - process_index_start, 'HH24:MI:SS');
		end loop;

		perform dblink_disconnect('db');

	exception when others then
		perform dblink_disconnect('db');

		GET STACKED DIAGNOSTICS err_context = PG_EXCEPTION_CONTEXT;
		RAISE INFO 'Error Name:%', SQLERRM;
		RAISE INFO 'Error State:%', SQLSTATE;
		RAISE INFO 'Error Context:%', err_context;
	end;

	return processed;
end; $$;


-- run it
select reindex_by_bloat_percent('postgres://user:pass@localhost:5432/dbname', 'public', 20);

Warning: Must be run from different instance, it stucks in the transaction otherwise.