Re: ALTER COLUMN ... SET EXPRESSION to alter stored generated column's expression

Hi,

Currently, we have an option to drop the expression of stored generated columns
as:

ALTER [ COLUMN ] column_name DROP EXPRESSION [ IF EXISTS ]

But don't have support to update that expression. The attached patch provides
that as:

ALTER [ COLUMN ] column_name SET EXPRESSION expression

  AT_ColumnExpression, /* alter column drop expression */

+ COMPLETE_WITH("(", "COMPRESSION", "EXPRESSION", "DEFAULT", "GENERATED", "NOT NULL", "STATISTICS", "STORAGE",

+ bool isdrop = (cmd->def == NULL);

static ObjectAddress ATExecColumnExpression(AlteredTableInfo *tab, Relation rel,
     const char *colName, Node *newDefault,
     bool missing_ok, LOCKMODE lockmode);

/*
* Mark the column as no longer generated.  (The atthasdef flag needs to

AT_ColumnExpression, /* alter column drop expression */