Fixes #41 mq_clear_ must keep the nil message (#43)

migrations/20220713122907_fix-clear_all-keep-nil-message.down.sql

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+-- Add down migration script here

migrations/20220713122907_fix-clear_all-keep-nil-message.up.sql

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+CREATE OR REPLACE FUNCTION mq_clear(channel_names TEXT[])
+RETURNS VOID AS $$
+BEGIN
+    WITH deleted_ids AS (
+        DELETE FROM mq_msgs
+        WHERE channel_name = ANY(channel_names)
+          AND id != uuid_nil()
+        RETURNING id
+    )
+    DELETE FROM mq_payloads WHERE id IN (SELECT id FROM deleted_ids);
+END;
+$$ LANGUAGE plpgsql;
+COMMENT ON FUNCTION mq_clear IS
+    'Deletes all messages with corresponding payloads from a list of channel names';
+
+
+CREATE OR REPLACE FUNCTION mq_clear_all()
+RETURNS VOID AS $$
+BEGIN
+    WITH deleted_ids AS (
+        DELETE FROM mq_msgs
+        WHERE id != uuid_nil()
+        RETURNING id
+    )
+    DELETE FROM mq_payloads WHERE id IN (SELECT id FROM deleted_ids);
+END;
+$$ LANGUAGE plpgsql;
+COMMENT ON FUNCTION mq_clear_all IS
+    'Deletes all messages with corresponding payloads';