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4D v14

Sets and named selections

In addition to the manipulations explained in the video, you should also note that:

In terms of memory:

a selection needs 4 bytes per selection record, regardless of the number of records in the table (unlike sets).
A set requires 1 bit for each record in the table.
Sets are composed of a series of bits. A set contains as many bits as there are records in the table. Each bit of a set corresponds to a quick report “included in the selection” or “not included” for the Xth record of the table at the moment the set is composed.

The table below sums up the all of possibilities for sets and temporary selections:

Theme	Set	Named Selection
Memory space for a record	1 bit	4 bytes
Keep order by	No	Yes
Keep the current record	No	Yes
Union	Yes	No
Intersection	Yes	No
Difference	Yes	No
Save to disk	Yes	No
Memory size of a selection of 10 records out of 20,000	20,000 bits or 2500 bytes	10 x 4 bytes = 40 bytes
Scope	Local, Process, Interprocess	Process, Interprocess

You can only compare sets from the same table.

Warning: The behavior of a set implies during a limited time and eventually using semaphores (see 4D documentation for more information).

In fact, a set makes a bit correspond to the physical position of each table record. In case of deleting and then adding records, the former content of a physical record could be replaced by the new content that is no longer in line with what the set is supposed to represent.

Be methodical in the use of sets, which remain an efficient and fast way to compare selections.

To keep a selection, there’s a third solution that involves using an array that contains your identification using the SELECTION TO ARRAY command.

You can use a fourth solution using stored clusters; for example, in BLOBs.

Of course, when selections are no longer necessary, you can free up memory by erasing them.

 CLEAR SET("SetName")

 CLEAR NAMED SELECTION("SelectionName")

Video script