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The following bug has been logged on the website:

Bug reference:      16038
Logged by:          death lock
Email address:      deathlock13@gmail.com
PostgreSQL version: 12.0
Operating system:   WinXP (VisualStudio 2013) , Debian11 gcc9
Description:        

create schema if not exists test ;
drop table test.testa;
CREATE TABLE test.testa (
    code character varying(2) NOT NULL,
    namez character varying(255) NOT NULL,
    descr character varying(1024),
    wersja numeric(1,0),
    modt timestamp without time zone,
    crtt timestamp without time zone,
    crt integer,
    mod integer,
    -- ida numeric(10,0) DEFAULT nextval(('test.sq_testa'::text)::regclass)
NOT NULL,
    -- fk_tmp character varying(20),
    CONSTRAINT test_pk PRIMARY KEY (code)
);

INSERT INTO test.testa VALUES ('0', 'AA', NULL, NULL, '2018-02-19
09:40:38.776875', '2008-05-20 10:06:28.171', 1001, 1001 /*,  1, 'DUMMYA'
*/);
-- INSERT INTO test.testa VALUES ('1', 'BB', NULL, NULL, '2018-02-19
09:40:38.776875', '2008-05-20 10:06:28.171', 1001, 1001 /*,  2, 'DUMMYB'
*/);

drop sequence if exists test.sq_testb;
create SEQUENCE test.sq_testb;

alter table test.testa
add column idb numeric(10,0) NOT NULL DEFAULT nextval('test.sq_testb'),
add column fk_tmpb varchar(20);

server process (PID 21884) was terminated by signal 11: Segmentation fault
- empty table - alter goes ok , split alter into 2 add col - works too