README.md in fluent-plugin-sql-0.2.1 vs README.md in fluent-plugin-sql-0.2.2

@@ -1,21 +1,21 @@
 # SQL input plugin for Fluentd event collector
 
 ## Overview
 
-This sql input plugin reads records from a RDBMS periodically. Thus you can replicate tables to other storages through Fluentd.
+This sql input plugin reads records from a RDBMS periodically. Thus you can copy tables to other storages through Fluentd.
 
 ## How does it work?
 
-This plugin runs following SQL repeatedly every 60 seconds to *tail* a table like `tail` command of UNIX.
+This plugin runs following SQL periodically:
 
 SELECT * FROM *table* WHERE *update\_column* > *last\_update\_column\_value* ORDER BY *update_column* ASC LIMIT 500
 
-What you need to configure is *update\_column*. The column needs to be updated every time when you update the row so that this plugin detects newly updated rows. Generally, the column is a timestamp such as `updated_at`.
-If you omit to set the column, it uses primary key. And this plugin can't detect updated but it only reads newly inserted rows.
+What you need to configure is *update\_column*. The column should be an incremental column (such as AUTO\_ INCREMENT primary key) so that this plugin reads newly INSERTed rows. Alternatively, you can use a column incremented every time when you update the row (such as `last_updated_at` column) so that this plugin reads the UPDATEd rows as well.
+If you omit to set *update\_column* parameter, it uses primary key.
 
-It stores last selected rows to a file named state\_file to not forget the last row when fluentd restarted.
+It stores last selected rows to a file (named *state\_file*) to not forget the last row when Fluentd restarts.
 
 ## Configuration
 
     <source>
       type sql
@@ -24,29 +24,29 @@
       database rdb_database
       adapter mysql2_or_postgresql_etc
       user myusername
       password mypassword
 
-      tag_prefix my.rdb
+      tag_prefix my.rdb  # optional, but recommended
 
-      select_interval 60s
-      select_limit 500
+      select_interval 60s  # optional
+      select_limit 500     # optional
 
       state_file /var/run/fluentd/sql_state
 
       <table>
-        tag table1
         table table1
+        tag table1  # optional
         update_column update_col1
-        time_column time_col2
+        time_column time_col2  # optional
       </table>
 
       <table>
-        tag table2
         table table2
+        tag table2  # optional
         update_column updated_at
-        time_column updated_at
+        time_column updated_at  # optional
       </table>
 
       # detects all tables instead of <table> sections
       #all_tables
     </source>
@@ -65,8 +65,13 @@
 
 \<table\> sections:
 
 * **tag** tag name of events (optional; default value is table name)
 * **table** RDBM table name
-* **update_column**
-* **time_column** (optional)
+* **update_column**: see above description
+* **time_column** (optional): if this option is set, this plugin uses this column's value as the the event's time. Otherwise it uses current time.
 
+## Limitation
+
+You should make sure target tables have index (and/or partitions) on the *update\_column*. Otherwise SELECT causes full table scan and serious performance problem.
+
+You can't replicate DELETEd rows.