feat: add trigram index to tags (#6615)

Fixes: chatwoot/product#796

This migration adds a trigram index to the tags table on the name column. This is used to speed up the search for tags by name, this is a heavy sub-query when generating reports where labels are involved.

Trigram indexes are used to speed up LIKE queries; they are not used for equality queries. This is because the index is not a btree index, it is a GIN index. This means that the index is not ordered and so cannot be used for equality queries.

Read more: https://www.postgresql.org/docs/current/pgtrgm.html

The index is created concurrently: https://thoughtbot.com/blog/how-to-create-postgres-indexes-concurrently-in
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Co-authored-by: Sojan <sojan@pepalo.com>

db/migrate/20230306100327_add_trgm_index_to_tag_name.rb

@@ -0,0 +1,21 @@
+# This migration adds a trigram index to the tags table on the name column.
+# This is used to speed up the search for tags by name, this is a heavy
+# sub query when genereating reports where labels are involved.
+#
+# Trigram indexes are used to speed up LIKE queries, they are not used for
+# equality queries. This is because the index is not a btree index, it is a
+# GIN index. This means that the index is not ordered, and so cannot be used
+# for equality queries.
+#
+# Read more: https://www.postgresql.org/docs/current/pgtrgm.html
+
+class AddTrgmIndexToTagName < ActiveRecord::Migration[6.1]
+  # disabling ddl transaction is required for concurrent index creation
+  # https://thoughtbot.com/blog/how-to-create-postgres-indexes-concurrently-in
+  disable_ddl_transaction!
+
+  def change
+    add_index :tags, 'LOWER(name) gin_trgm_ops', using: :gin, name: 'tags_name_trgm_idx', algorithm: :concurrently
+    # resolves to CREATE INDEX tags_name_trgm_idx ON public.tags USING gin (lower((name)::text) gin_trgm_ops);
+  end
+end