Draft: refactor follows and update_nofications indexing

Based on !819

hive/db/sql_scripts/hive_post_operations.sql

@@ -351,6 +351,71 @@ END
 $function$
 ;
 
+DROP FUNCTION IF EXISTS hivemind_app.process_hive_post_mentions;
+CREATE OR REPLACE FUNCTION hivemind_app.process_hive_post_mentions(_post_id hivemind_app.hive_posts.id%TYPE)
+RETURNS SETOF BIGINT
+LANGUAGE plpgsql
+AS
+$function$
+BEGIN
+  -- With clause is inlined, modified call to reptracker_endpoints.get_account_reputation.
+  -- Reputation is multiplied by 7.5 rather than 9 to bring the max value to 100 rather than 115.
+  -- In case of reputation being 0, the score is set to 25 rather than 0.
+  RETURN query
+    WITH log_account_rep AS
+    (
+        SELECT
+            account_id,
+            LOG(10, ABS(nullif(reputation, 0))) AS rep,
+            (CASE WHEN reputation < 0 THEN -1 ELSE 1 END) AS is_neg
+        FROM reptracker_app.account_reputations
+    ),
+    calculate_rep AS
+    (
+        SELECT
+            account_id,
+            GREATEST(lar.rep - 9, 0) * lar.is_neg AS rep
+        FROM log_account_rep lar
+    ),
+    final_rep AS
+    (
+        SELECT account_id, (cr.rep * 7.5 + 25)::INT AS rep FROM calculate_rep AS cr
+    ),
+    mentions AS
+    (
+        SELECT DISTINCT _post_id AS post_id, T.author_id, ha.id AS account_id, T.block_num
+        FROM
+          hivemind_app.hive_accounts ha
+        INNER JOIN
+        (
+          SELECT LOWER( ( SELECT trim( T.mention::text, '{""}') ) ) AS mention, T.author_id, T.block_num
+          FROM
+          (
+            SELECT
+              hp.id, REGEXP_MATCHES( hpd.body, '(?:^|[^a-zA-Z0-9_!#$%&*@\\/])(?:@)([a-zA-Z0-9\\.-]{1,16}[a-zA-Z0-9])(?![a-z])', 'g') AS mention, hp.author_id, hp.block_num
+            FROM hivemind_app.hive_posts hp
+            INNER JOIN hivemind_app.hive_post_data hpd ON hp.id = hpd.id
+            WHERE hp.id = _post_id
+          ) AS T
+        ) AS T ON ha.name = T.mention
+        WHERE ha.id != T.author_id
+        ORDER BY T.block_num, ha.id
+    )
+    INSERT INTO hivemind_app.hive_notification_cache
+    (block_num, type_id, created_at, src, dst, dst_post_id, post_id, score, payload, community, community_title)
+    SELECT hm.block_num, 16, (SELECT hb.created_at FROM hivemind_app.blocks_view hb WHERE hb.num = (hm.block_num - 1)) AS created_at, hm.author_id, hm.account_id, hm.post_id, hm.post_id, COALESCE(rep.rep, 25), '', '', ''
+    FROM mentions AS hm
+    JOIN hivemind_app.hive_accounts AS a ON hm.account_id = a.id
+    LEFT JOIN final_rep AS rep ON a.haf_id = rep.account_id
+    WHERE hm.block_num > hivemind_app.block_before_irreversible( '90 days' )
+        AND COALESCE(rep.rep, 25) > 0
+    ORDER BY hm.block_num, created_at, hm.author_id, hm.account_id
+    ON CONFLICT (src, dst, post_id) WHERE type_id=16 DO UPDATE
+    SET block_num=EXCLUDED.block_num, created_at=EXCLUDED.created_at
+    RETURNING id;
+END;
+$function$;
+
 DROP FUNCTION IF EXISTS hivemind_app.delete_hive_post(character varying,character varying,character varying, integer, timestamp)
 ;
 CREATE OR REPLACE FUNCTION hivemind_app.delete_hive_post(