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Casting a value that's already of a type with a specific typmod to an unspecified typmod doesn't do anything so far as run-time behavior is concerned. However, it really ought to change the exposed type of the expression to match. Up to now, coerce_type_typmod hasn't bothered with that, which creates gotchas in contexts such as recursive unions. If for example one side of the union is numeric(18,3), but it needs to be plain numeric to match the other side, there's no direct way to express that. This is easy enough to fix, by inserting a RelabelType to update the exposed type of the expression. However, it's a bit nervous-making to change this behavior, because it's stood for a really long time. (I strongly suspect that it's like this in part because the logic pre-dates the introduction of RelabelType in 7.0. The commit log message for 57b30e8e2 is interesting reading here.) As a compromise, we'll sneak the change into 14beta3, and consider back-patching to stable branches if no complaints emerge in the next three months. Discussion: https://postgr.es/m/CABNQVagu3bZGqiTjb31a8D5Od3fUMs7Oh3gmZMQZVHZ=uWWWfQ@mail.gmail.com
src/backend/parser/README Parser ====== This directory does more than tokenize and parse SQL queries. It also creates Query structures for the various complex queries that are passed to the optimizer and then executor. parser.c things start here scan.l break query into tokens scansup.c handle escapes in input strings gram.y parse the tokens and produce a "raw" parse tree analyze.c top level of parse analysis for optimizable queries parse_agg.c handle aggregates, like SUM(col1), AVG(col2), ... parse_clause.c handle clauses like WHERE, ORDER BY, GROUP BY, ... parse_coerce.c handle coercing expressions to different data types parse_collate.c assign collation information in completed expressions parse_cte.c handle Common Table Expressions (WITH clauses) parse_expr.c handle expressions like col, col + 3, x = 3 or x = 4 parse_func.c handle functions, table.column and column identifiers parse_node.c create nodes for various structures parse_oper.c handle operators in expressions parse_param.c handle Params (for the cases used in the core backend) parse_relation.c support routines for tables and column handling parse_target.c handle the result list of the query parse_type.c support routines for data type handling parse_utilcmd.c parse analysis for utility commands (done at execution time) See also src/common/keywords.c, which contains the table of standard keywords and the keyword lookup function. We separated that out because various frontend code wants to use it too.