Serge Rielau created SPARK-40822: ------------------------------------ Summary: Use stable derived-column-alias algorithm, suitable for CREATE VIEW Key: SPARK-40822 URL: https://issues.apache.org/jira/browse/SPARK-40822 Project: Spark Issue Type: Improvement Components: Spark Core Affects Versions: 3.4.0 Reporter: Serge Rielau
Spark has the ability derive column aliases for expressions if no alias was provided by the user. E.g. CREATE TABLE T(c1 INT, c2 INT); SELECT c1, `(c1 + 1)`, c3 FROM (SELECT c1, c1 + 1, c1 * c2 AS c3 FROM T); This is a valuable feature. However, the current implementation works by pretty printing the expression from the logical plan. This has multiple downsides: * The derived names can be unintuitive. For example the brackets in `(c1 + 1)` or outright ugly, such as: SELECT `substr(hello, 1, 2147483647)` FROM (SELECT substr('hello', 1)) AS T; * We cannot guarantee stability across versions since the logical lan of an expression may change. The later is a major reason why we cannot allow CREATE VIEW without a column list except in "trivial" cases. CREATE VIEW v AS SELECT c1, c1 + 1, c1 * c2 AS c3 FROM T; Not allowed to create a permanent view `spark_catalog`.`default`.`v` without explicitly assigning an alias for expression (c1 + 1). There are two way we can go about fixing this: # Stop deriving column aliases from the expression. Instead generate unique names such as `_col_1` based on their position in the select list. This is ugly and takes away the "nice" headers on result sets # Move the derivation of the name upstream. That is instead of pretty printing the logical plan we pretty print the lexer output, or a sanitized version of the expression as typed. The statement as typed is stable by definition. The lexer is stable because i has no reason to change. And if it ever did we have a better chance to manage the change. In this feature we propose the following semantic: # If the column alias can be trivially derived (some of these can stack), do so: ** a (qualified) column reference => the unqualified column identifier cat.sch.tab.col => col ** A field reference => the fieldname struct.field1.field2 => field2 ** A cast(column AS type) => column cast(col1 AS INT) => col1 ** A map lookup with literal key => keyname map.key => key map['key'] => key ** A parameter less function => unqualified function name current_schema() => current_schema # Take the lexer tokens of the expression, eliminate comments, and append them. foo(tab1.c1 + /* this is a plus*/ 1) => `foo(tab1.c1+1)` Of course we wan this change under a config. If the config is set we can allow CREATE VIEW to exploit this and use the derived expressions. PS: The exact mechanics of formatting the name is very much debatable. E.g.spaces between token, squeezing out comments - upper casing - preserving quotes or double quotes...) -- This message was sent by Atlassian Jira (v8.20.10#820010) --------------------------------------------------------------------- To unsubscribe, e-mail: issues-unsubscr...@spark.apache.org For additional commands, e-mail: issues-h...@spark.apache.org