PostgreSQL – How to Use pgloader to Transfer SQLite to Postgres with Uppercase Table Names

Your second approach is not normalized: adding another sensor would require creating a new table, and querying multiple sensors would be horribly complex.

When all four sensors get a new value every second, and if you have multiple tables, then the database would have to update four tables. A single table is certainly more efficient.

For fast queries on time ranges, you need an index on the timestamp column. This implies that every INSERT also needs to update the index.
(If you want fast queries on time ranges for a single sensor, you need another index that contains both the sensor_id and timestamp columns.)

Please note that you do not need AUTOINCREMENT when you never delete rows. (Autoincrement In SQLite)

You explain:

I get result 1453 only when all items in column attributeY are empty for 1453.

But that's incorrect. Bold emphasis mine. The aggregate function count returns (per documentation):

number of input rows for which the value of expression is not null

The same is true for SQLite (per documentation):

The count(X) function returns a count of the number of times that X is not NULL in a group.

You obviously have one or more rows with a non-null value in attributeY in your Postgres table - probably an empty string ''.

Test with:

SELECT *
FROM   myschema.table
WHERE  attributeX = 1453
AND    attributeY IS NOT NULL;

Be sure to understand the difference between "empty" ('') and NULL:

Empty strings are character types (typically text, varchar or char) with 0 characters ('') - so basically "nothing", the equivalent of 0 for a numeric data type. NULL is possible for any data type and means "unknown". Some clients have a hard time making the difference clear in their display.

Alternative query

To find attributeX where all attributeY are either empty or NULL use this alternative query:

SELECT attributeX
FROM   myschema.table
GROUP  BY attributeX
HAVING NOT bool_or(attributeY <> '');

The expression attributeY <> '' is only true for non-null, non-empty attributeY. The aggregate function bool_or returns (per documentation):

true if at least one input value is true, otherwise false

Those are the results we exclude (NOT ...) and return the rest. Voilá.

There are many other (less elegant) ways to achieve the same. Like:

... HAVING count(CASE WHEN attributeY <> '' THEN 1 END) = 0;
... HAVING count(attributeY <> '' OR NULL) = 0;
... HAVING count(NULLIF(attributeY, '') = 0;

More:

• For absolute performance, is SUM faster or COUNT?
• Replace empty strings with null values