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Postgressql 中的表达式短切表达式评价

原标题：Expression short cut evaluation in Postgresql

In C, the classic short cut evalution might be something like if ( a != NULL && a->b == 0 )... to avoid evaluating a structure reference if invalid... I m trying something similar in a postgresql trigger: IF TG_TABLE_NAME::text != collection_item OR OLD.domain = HERB OR NEW.domain = HERB THEN ... blah END IF; So if the table is collection_item it contains the field domain, but other tables don t have that field. So in theory if table is not collection_item, it should short circuit and not evaluate OLD.domain. However this wasn t working, it would give an error that `ERROR: record "old" has no field "domain" So I read here about evaluation rules, https://www.postgresql.org/docs/9.0/sql-expressions.html#SYNTAX-EXPRESS-EVAL It implies that if you encounter this problem you can get around it with CASE. So I tried that... IF (CASE WHEN TG_TABLE_NAME::text != collection_item THEN true WHEN OLD.domain = HERB OR NEW.domain = HERB THEN true ELSE false END) THEN ... blah END IF; but I still get ``ERROR: record "old" has no field "domain"` I can get around it with nesting IFs... IF TG_TABLE_NAME::text = collection_item THEN IF OLD.domain = HERB OR NEW.domain = HERB THEN is_target := true; END IF; ELSE is_target := true; END IF; But I m curious why my other attempts don t work. I m expecting short cut evalution to work.

问题回答

The expressions in PL/pgSQL are "compiled" before execution. The compilation is transformation from text form to AST form. In this form it is interpreted by expression executor. In your case, the compilation stage fails. It should be fully completed without dependency on result of any operator or expression. The expression compilation is done immediately before first expression execution (evaluation) inside session. PLpgSQL uses similar steps. Before first execution in the session, the source code is "compiled" to AST form. AST is stored to session cache, and interpreted. In this stage, the expressions are not compiled. Expressions are compiled just before own execution.

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