The following SQL query shows how different can be the result of concatenating arrays with NULL values:
select '{e1,e2}'::text[] || 't'::text as normal_array_concatenation, NULL::text[] || 't'::text as appending_element_to_existing_NULL_value_array, NULL || 't'::text as appending_element_to_NULL_value, '{e1,e2}'::text[] || NULL::text as appending_typed_NULL, '{e1,e2}'::text[] || NULL::text[] as appending_typed_NULL_array;
The result of the execution (on 9.0) is:
─[ RECORD 1 ]──────────────────────────────────┬───────────── normal_array_concatenation │ {e1,e2,t} appending_element_to_existing_null_value_array │ {t} appending_element_to_null_value │ appending_typed_null │ {e1,e2,NULL} appending_typed_null_array │ {e1,e2}
That explains why one can simply initialize a new array variable in PL/pgSQL and then immediately start concatenating it with values, not pre-initializing it with an empty array (that you have to do when you want to populate a text string... you cannot just take a NULL::text variable and start concatenating strings to it, you have to first pre-inizialize it with en empty string, but with arrays you can).
Another very important issue, that is related to that beheviour of arrays, is that when creating dynamic SQL queries for EXECUTE command in PL/pgSQL you SHOULD always put explicit type casts to the variables that you quote using quote_literal() or quote_nullable() when building dynamic queries:
DO $SQL$ DECLARE r text[]; t text := NULL; a text[] := NULL; BEGIN SELECT '{e1,e2}'::text[] || a INTO r; RAISE INFO 'array concatenate: r is %', r; EXECUTE $$SELECT '{e1,e2}'::text[] || $$ || quote_nullable(a) || $$::text[] $$ INTO r; RAISE INFO 'array concatenate from EXECUTE: r is %', r; SELECT '{e1,e2}'::text[] || t INTO r; RAISE INFO 'array append: r is %', r; EXECUTE $$SELECT '{e1,e2}'::text[] || $$ || quote_nullable(t) || $$::text $$ INTO r; RAISE INFO 'array append from EXECUTE: r is %', r; /* These 2 examples will fail on the runtime throwing exception (operator is not unique: text[] || unknown) --EXECUTE $$SELECT '{e1,e2}'::text[] || $$ || quote_nullable(a) INTO r; --RAISE INFO 'r is %', r; -- --EXECUTE $$SELECT '{e1,e2}'::text[] || $$ || quote_nullable(t) INTO r; --RAISE INFO 'r is %', r; */ END; $SQL$;
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