Details
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Improvement
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Status: Resolved
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Major
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Resolution: Fixed
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3.4.0
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None
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Important
Description
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
- a (qualified) column reference => the unqualified column identifier
- 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...)
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