[GitHub] [flink] dawidwys commented on a change in pull request #10110: [FLINK-14301][doc] add documentation for functions categories and new function resolution orders

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dawidwys commented on a change in pull request #10110: [FLINK-14301][doc] add documentation for functions categories and new function resolution orders
URL: https://github.com/apache/flink/pull/10110#discussion_r345321063
 
 

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 File path: docs/dev/table/functions/index.md
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 @@ -0,0 +1,96 @@
+---
+title: "Functions"
+nav-id: table_functions
+nav-parent_id: tableapi
+nav-pos: 116
+is_beta: true
+nav-show_overview: true
+---
+<!--
+Licensed to the Apache Software Foundation (ASF) under one
+or more contributor license agreements.  See the NOTICE file
+distributed with this work for additional information
+regarding copyright ownership.  The ASF licenses this file
+to you under the Apache License, Version 2.0 (the
+"License"); you may not use this file except in compliance
+with the License.  You may obtain a copy of the License at
+
+  http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing,
+software distributed under the License is distributed on an
+"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+KIND, either express or implied.  See the License for the
+specific language governing permissions and limitations
+under the License.
+-->
+
+Flink Table API & SQL empowers users to do data transformations with functions.
+
+Types of Functions
+------------------
+
+There are two dimensions to classify functions in Flink.
+
+One dimension is system (or built-in) functions v.s. catalog functions. System functions have no namespace and can be
+referenced with just their names. Catalog functions belong to a catalog and database therefore they have catalog and database
+namespaces, they can be referenced by either fully/partially qualified name (`catalog.db.func` or `db.func`) or just the
+function name.
+
+The other dimension is temporary functions v.s. persistent functions. Temporary functions are volatile and only live up to
+ lifespan of a session, they are always created by users. Persistent functions live across lifespan of sessions, they are either
+ provided by the system or persisted in catalogs.
+ 
+The two dimensions give Flink users 4 categories of functions:
+
+1. Temporary system functions
+2. System functions
+3. Temporary catalog functions
+4. Catalog functions
+
+Referencing Functions
+---------------------
+
+There are two ways users can reference a function in Flink - referencing function precisely or ambiguously.
+
+## Precise Function Reference
+
+Precise function reference empowers users to use catalog functions specifically, and across catalog and across database, 
+e.g. `select mycatalog.mydb.myfunc(x) from mytable` and `select mydb.myfunc(x) from mytable`.
+
+This is only supported starting from Flink 1.10.
+
+## Ambiguous Function Reference
+
+In ambiguous function reference, users just specify the function's name in SQL query, e.g. `select myfunc(x) from mytable`.
+
+This is the only supported option before Flink 1.10.
+
+
+Function Resolution Order
+-------------------------
+
+The resolution order only matters when there are functions of different types but the same name, 
+e.g. when there’re three functions all named “myfunc” but are of temporary catalog, catalog, and system function respectively. 
+If there’s no function name collision, functions will just be resolved to the sole one.
+
+## Precise Function Reference
+
+Because system functions don’t have namespaces, a precise function reference in Flink must be pointing to either a temporary catalog 
+function or a catalog function.
+
+The resolution order is:
+
+1. Temporary catalog function
+2. Catalog function
+
+## Ambiguous Function Reference
+
+In Flink 1.10, we also redefined its function resolution order to be:
+
+The new order is:
 
 Review comment:
   I agree with @sjwiesman. Personally I think the documentation should just describe the current state. Unless this is a disruptive change that requires users to change their code. I don't think this is the case. I
   
   f I am wrong, and it does, I would mention the version, but also would mention what are the actions that a user must take and would change the title to something along the lines: "Migration guide/Migration"

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