pali sentence sim add index

db/postgresql/migrations/2021-11-28-093615_sent_sim_index_id/down.sql

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+-- This file should undo anything in `up.sql`
+
+DROP INDEX  IF EXISTS sent_sim_index_id ;

db/postgresql/migrations/2021-11-28-093615_sent_sim_index_id/up.sql

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+-- Your SQL goes here
+CREATE INDEX sent_sim_index_id ON sent_sim_index (sent_id);