PostgreSQL trigger to generate codes for multiple tables dynamically

问题:

I'd like to generate codes for many tables in the database, and stopped to refactor my solution when I was ready to write my third implementation of "get code for table X".

My code is this:

-- Tenants receive a code that's composed of a portion of their subdomain and a unique number. -- This number comes from this sequence. CREATE SEQUENCE tenant_codes_seq MAXVALUE 9999 NO CYCLE;  CREATE TABLE tenants (     subdomain   varchar(36) NOT NULL UNIQUE   , tenant_code char(8)     NOT NULL UNIQUE   , PRIMARY KEY (tenant_code) );  -- This function expects four parameters: -- 1. The column that's receiving the generated code (RECEIVING_COLUMN_NAME) -- 2. The column that's used to salt the code (SALT_COLUMN_NAME) -- 3. The number of characters to use from the salt column (SALT_LENGTH) -- 4. The sequence name, but defaults to RECEIVING_COLUMN_NAME || 's' CREATE OR REPLACE FUNCTION generate_table_code() RETURNS trigger AS $$ DECLARE   receiving_column_name text;   salt_column_name      text;   salt_length           text;   sequence_name         text; BEGIN   receiving_column_name := TG_ARGV[0];   salt_column_name      := TG_ARGV[1];   salt_length           := TG_ARGV[2];    CASE   WHEN TG_NARGS = 3 THEN     sequence_name := receiving_column_name || 's';   WHEN TG_NARGS = 4 THEN     sequence_name := TG_ARGV[3];   ELSE     RAISE EXCEPTION '3 or 4 arguments expected, received %', TG_NARGS;   END CASE;    -- The intent is to return ABC-0001 when salt_column contains 'ABC'   EXECUTE 'rpad(substr('                ||           quote_ident(salt_column_name) ||           ', 1, 4), 4, '                ||           quote_literal('-')            ||           ') || lpad(nextval('          ||           quote_literal(sequence_name)  ||           ')::text, '                   ||           quote_literal(salt_length)    ||           ', '                          ||           quote_literal('0')            ||           ')'   INTO STRICT NEW;   RETURN NEW; END $$ LANGUAGE plpgsql;  CREATE TRIGGER generate_tenant_code_trig   BEFORE INSERT ON tenants FOR EACH ROW   EXECUTE PROCEDURE generate_table_code('tenant_code', 'subdomain', 4); 

How do I assign to NEW.tenant_code, NEW.user_code or NEW.table_whatever_code?

Running some tests yields the correct "statement", but I can't seem to assign correctly:

INSERT INTO tenants(subdomain) VALUES ('abc')   CREATE TABLE ERROR:  syntax error at or near "NEW" LINE 1: NEW.tenant_code := rpad(substr(subdomain, 1, 4), 4, '-') || ...         ^ QUERY:  NEW.tenant_code := rpad(substr(subdomain, 1, 4), 4, '-') || lpad(nextval('tenant_codes')::text, '4', '0'::text) CONTEXT:  PL/pgSQL function "generate_table_code" line 20 at EXECUTE statement 

回答1:

I'd be quite enthusiastic to be shown wrong (I occasionally need this myself too), but best I'm aware, referring column names using variables is one of those cases where you actually need to use PL/C triggers rather than PL/PgSQL triggers. You'll find examples of such triggers in contrib/spi and on PGXN.

Alternatively, name your columns consistently so as to be able to reference them directly, e.g. NEW.tenant_code.

Personally, I generally end up writing a function that creates the trigger:

create function create_tg_stuff(_table regclass, _args[] text[])   returns void as $$ begin   -- explore pg_catalog a bit   execute $x$   create function $x$ || quote_ident(_table || '_tg_stuff') || $x$()     returns trigger as $t$   begin     -- more stuff     return new;   end;   $t$ language plpgsql;   $x$; end; $$ language plpgsql; 

回答2:

NEW is type RECORD, so you can't assign to that AFAIK.

To set the value of a column, assign to NEW.column, for example:

NEW.tenant_code := (SELECT some_calculation); 

Maybe your design is too complicated; PL/SQL is a very limited language - try to make your code as simple as possible

文章来源: PostgreSQL trigger to generate codes for multiple tables dynamically