Today I realized a number of points where PostgreSQL (v10.3) is rather lackluster and sparse in its error messages.
The first point is illustrated by this code:
drop schema if exists X cascade; create schema X; create domain X.an_illegal_regex as text check ( value ~ '(' ); create table X.table_with_illegal_constraint ( a text, constraint "column a must have a bogus value" check ( a::X.an_illegal_regex = a ) ); select * from X.table_with_illegal_constraint; insert into X.table_with_illegal_constraint values ( 'xxx' ), -- ( 'xxx' ), ( 'foo' ), ( 'xyx' );
This code will throw with
psql:db/experiments/pg-error-fail-illegal-regex.sql:17: ERROR: invalid regular expression: parentheses () not balanced
There are several problems with this error message:
FAILURE: the error is really in line 5 where a syntactically invalid
RegEx is created; the fact that it is a RegEx and not a general
string is obvious from the semantics of the
~ (tilde) operator at
that point in time.
FAILURE: the offending RegEx is not referred to and not quoted in the error message. As such, it could be anywhere in my many, many kLOCs big DB definition. I cannot even search the RegEx with a RegEx because all I know is some parenthesis is missing, somewhere: RegExes cannot match parentheses, and PG RegExes do not have a unique syntactic marker to them.
FAILURE: before the
insert statement, everything runs dandy. We could have
built an entire data warehouse application on top of a table definition
that can never be syntactically processed but which will only fail when
someone accidentally tries to insert a line.
FAILURE: I can select from a table with a syntactically invalid definition.
The second point is related:
drop schema if exists X cascade; create schema X; create domain X.a_legal_regex as text check ( value ~ '^x' ); create table X.table_with_constraints ( a text, constraint "column a must start with x" check ( a::X.a_legal_regex = a ), constraint "field b must have 3 characters" check ( character_length( a ) = 3 ) ); insert into X.table_with_constraints values ( 'xxx' ), ( 'foo' ), /* A: violates first constraint */ -- ( 'xxxx' ), /* B: violates second constraint */ ( 'xyx' );
With only line B active, this gives:
psql:db/experiments/pg-error-fail-no-constraint-name.sql:16: ERROR: new row for relation "table_with_constraints" violates check constraint "field b must have 3 characters" DETAIL: Failing row contains (xxxx).
SUCCESS: we get the name of the relation and the name of the violated rule.
SUCCESS: the offending piece of data is quoted.
FAILURE: we don't get the full name of the relation, which is "X"."table_with_constraints". Neither do we get the name of the column that received the offending value.
Lastly, with only line A (not line B) active:
psql:db/experiments/pg-error-fail-no-constraint-name.sql:16: ERROR: value for domain x.a_legal_regex violates check constraint "a_legal_regex_check"
FAILURE: no reference to the affected table, column is made.
FAILURE: no reference to the offending piece of data is made.
FAILURE: no reference to the offended constraint is made ("column a must start with x").
What are the best practices or workarounds for the above shortcomings? I've been trying for several hours to figure out what causes an error message a la
value for domain xxx violates check constraint "xxx_check" by rewriting table definitions,
inserting data row by row and so on, to no avail. What I need is a full chain of the objects (column -> table -> constraint -> domain -> check) that are involved in the error.