PostgreSQL: Why "select into" is faster than "select count" from the same sourcetable?

After upgrading PG from 11 to 15, I struggeling with the following situation:

old (working) Setup: Postgres 11 PostGIS 2.5

Setup: Postgres 15 PostGIS 3.3

table_1 (~14.000.000 rows)

When I do select count(*) from table_1 it never seems to be finished. When I do select * into table_2 from table_1 and than select count(*) from table_2, it would finished in ~15 minutes.

The concrete statements are:

-- this never stops
select count(*)
FROM table_1 t1
INNER JOIN table_1 t2 ON (ST_Equals(t1.wkb_geometry, t2.wkb_geometry))
WHERE t1.gid > t2.gid
;

-- this works
select *
into table_2
from table_1 
;
select count(*)
FROM table_2 t1
INNER JOIN table_2 t2 ON (ST_Equals(t1.wkb_geometry, t2.wkb_geometry))
WHERE t1.gid > t2.gid
;

In both cases there is an index created on wkb_geometry, like this:

CREATE INDEX table_1_geom ON table_1 
USING gist (wkb_geometry)
TABLESPACE pgdata;

I already recreated the index an vacuum the table, but nothing worked for me.

Update: I figured out, that the problem is located at the pkey for column gid. After the select into... table_2 has no pkey on gid. If i remove that pkey on table_1 it would work, too. So what happend by adding a pkey different in PG version 15? Any ideas?

Update 2: DDL/SQL for table_1

SELECT ST_MakeLine(sp,ep) AS wkb_geometry, 
         ogc_fid,
         ...
    INTO table_1
    FROM
       (SELECT
          ST_PointN(geom, generate_series(1, ST_NPoints(geom)-1)) as sp,
          ST_PointN(geom, generate_series(2, ST_NPoints(geom)  )) as ep,
          ogc_fid,
          ...
        FROM
           (SELECT (ST_Dump(ST_Boundary(wkb_geometry))).geom, 
                   ogc_fid,
                   ...
           FROM another_table
           ) AS linestrings
        ) AS segments;
    
    ALTER TABLE table_1 ADD COLUMN gid serial NOT NULL;
    --ALTER TABLE table_1 ADD CONSTRAINT table_1_pk PRIMARY KEY (gid);
    CREATE INDEX table_1_geom ON table_1 USING gist (wkb_geometry) TABLESPACE pgdata;