This page points out differences in behavior between the PostgreSQL capabilities supported in Spanner and their open source PostgreSQL equivalents.
Float8 underflow
In some cases, open source PostgreSQL returns an error such as
ERROR: value out of range: underflow if the result of floating point math
would be truncated to 0. Numbers too close to zero that are not representable as
distinct from zero cause an underflow error.
The PostgreSQL interface doesn't return an error in these cases; it returns the truncated 0 value instead.
concat() function
In the PostgreSQL interface, the concat() function returns
NULL if any argument is NULL. However, in open source PostgreSQL,
this function ignores NULLs and returns a concatenation of all non-NULL
arguments, or an empty string if all arguments are NULL. For example:
-- Returns `abcdef` in open source PostgreSQL.
-- Returns NULL in the PostgreSQL interface.
select concat('abc', NULL, 'def');
The PostgreSQL interface and open source PostgreSQL have
identical behavior for the || operator with scalar (non-array) operands.
array_cat() function
In the PostgreSQL interface, the array_cat() function returns
NULL if any argument is NULL. However, in open source PostgreSQL, this
function ignores NULLs and returns a concatenation of all non-NULL array
arguments, or a NULL array if all arguments are NULL. The same is true for the
|| operator with array operands. For example:
-- Returns `{abc, def}` in open source PostgreSQL.
-- Returns NULL in the PostgreSQL interface.
select '{abc}'::text[] || NULL::text[] || '{def}'::text[];
array_to_string() function
In the PostgreSQL interface, the array_to_string(array text_array, delimiter text [, null_string text ])
function returns NULL if the null_string argument is NULL. However, in
open source PostgreSQL, this function ignores the NULL array
elements and returns a concatenation of all non-NULL elements as a string
representation if the null_string argument is NULL. For example:
-- Returns `a,b,c` in open source PostgreSQL.
-- Returns NULL in the PostgreSQL interface.
select array_to_string('{a,b,NULL,c}'::text[], ',', NULL);