Can I convert a bunch of boolean columns to a single bitmap in PostgreSQL?

Question

I'd like to convert a query such as:

SELECT BoolA, BoolB, BoolC, BoolD FROM MyTable;

Into a bitmask, where the bits are defined by the values above.

For example, if BoolA and BoolD were true, I'd want 1001 or 9.

I have something in mind to the effect of:

SELECT
   CASE WHEN BoolD THEN 2^0 ELSE 0 END +
   CASE WHEN BoolC THEN 2^1 ELSE 0 END +
   CASE WHEN BoolB THEN 2^2 ELSE 0 END +
   CASE WHEN BoolA THEN 2^3 ELSE 0 END
FROM MyTable;

But I'm not sure if this is the best approach and seems rather verbose. Is there an easy way to do this?

Answer 1

For a bitmask, the type bitstring would be the better choice. Could look like this then:

SELECT BoolD::int::bit
    || BoolC::int::bit
    || BoolB::int::bit
    || BoolA::int::bit
FROM tbl;

true converts to 1, false to 0. You can simply concatenate bits to a bitstring.

Cast bit(n) to integer

It seems you need an integer as result - there is a simple & fast way:

SELECT (BoolD::int::bit
     || BoolC::int::bit
     || BoolB::int::bit
     || BoolA::int::bit)::bit(4)::int
FROM tbl;

Be sure to read the fine print in the chapter "Bit String Functions and Operators" of the manual.

---

I came up with two more ideas and put together a quick test / reference with 10k rows to sum it all up.

Test setup:

CREATE TEMP TABLE t (boola bool, boolb bool, boolc bool, boold bool);
INSERT INTO t
SELECT random()::int::bool
     , random()::int::bool
     , random()::int::bool
     , random()::int::bool
FROM   generate_series(1,10000);

Demo:

SELECT  CASE WHEN boold THEN 1 ELSE 0 END
     + (CASE WHEN boolc THEN 1 ELSE 0 END << 1)
     + (CASE WHEN boolb THEN 1 ELSE 0 END << 2)
     + (CASE WHEN boola THEN 1 ELSE 0 END << 3) AS andriy

     ,  boold::int
     + (boolc::int << 1)
     + (boolb::int << 2)
     + (boola::int << 3) AS mike

     , (boola::int::bit
     || boolb::int::bit
     || boolc::int::bit
     || boold::int::bit)::bit(4)::int AS erwin1

     ,  boold::int
     | (boolc::int << 1)
     | (boolb::int << 2)
     | (boola::int << 3) AS erwin2

     , (((
       boola::int << 1)
     | boolb::int << 1)
     | boolc::int << 1)
     | boold::int        AS erwin3
FROM   t
LIMIT  15;

You could also use | (bitwise OR) instead of the + operator.
Individual test runs show basically the same performance for all five methods.

Answer 2

Maybe like this:

SELECT
  (CASE WHEN BoolA THEN 1 ELSE 0 END << 0) +
  (CASE WHEN BoolB THEN 1 ELSE 0 END << 1) +
  (CASE WHEN BoolC THEN 1 ELSE 0 END << 2) +
  (CASE WHEN BoolD THEN 1 ELSE 0 END << 3) AS BitMask
FROM MyTable;

where << is the bitwise shift left operator.

Answer 3

I came up with this approach as well. It's the most concise I could find short of writing a custom function. I'll accept this answer unless anyone has anything more clever.

SELECT
  (BoolD::int << 0) +
  (BoolC::int << 1) +
  (BoolB::int << 2) +
  (BoolA::int << 3)
from MyTable;