postgresql pageinspect what does t_infomask, t_infomask2 comparison with integers really mean?

Question

create or replace function heap_page(relname text, pageno integer)
    returns
    table ( ctid tid,state text,xmin text,xmax text,hhu text,hot text,t_ctid tid)
    as $$
    select
    (pageno,lp)::text::tid as ctid,
    case lp_flags
        when 0 then 'unused'
        when 1 then 'normal'
        when 2 then 'redirect to '|| lp_off
        when 3 then 'dead'
    end as state,
    t_xmin || case
    when (t_infomask & 256) > 0 then 'c'
    when (t_infomask & 512) > 0 then 'a'
    else ''
    end as xmin,
    t_xmax || case
    when (t_infomask & 1024) >0 then 'c'
    when (t_infomask & 2048) >0 then 'a'
    else ''
    end as xmax,
    case when (t_infomask2 & 16384) >0 then 't' end as hhu,
    case when (t_infomask2 & 32768) >0 then 't' end as hot,
    t_ctid
    from heap_page_items
    (get_raw_page(relname, pageno))
    order by lp
    $$ language sql;

https://www.postgresql.org/docs/current/pageinspect.html
https://www.postgresql.org/docs/current/functions-math.html
https://doxygen.postgresql.org/htup__details_8h_source.html
I can understand the manual bitwise operator & example.

But here I don't understand all the comparisons t_infomask and t_infomask2 with variant integer.

Answer 1

You can find that information in src/include/access/htup_details.h.

About t_infomask:

/*
 * information stored in t_infomask:
 */
#define HEAP_HASNULL            0x0001  /* has null attribute(s) */
#define HEAP_HASVARWIDTH        0x0002  /* has variable-width attribute(s) */
#define HEAP_HASEXTERNAL        0x0004  /* has external stored attribute(s) */
#define HEAP_HASOID_OLD         0x0008  /* has an object-id field */
#define HEAP_XMAX_KEYSHR_LOCK   0x0010  /* xmax is a key-shared locker */
#define HEAP_COMBOCID           0x0020  /* t_cid is a combo CID */
#define HEAP_XMAX_EXCL_LOCK     0x0040  /* xmax is exclusive locker */
#define HEAP_XMAX_LOCK_ONLY     0x0080  /* xmax, if valid, is only a locker */

 /* xmax is a shared locker */
#define HEAP_XMAX_SHR_LOCK  (HEAP_XMAX_EXCL_LOCK | HEAP_XMAX_KEYSHR_LOCK)

#define HEAP_LOCK_MASK  (HEAP_XMAX_SHR_LOCK | HEAP_XMAX_EXCL_LOCK | \
                         HEAP_XMAX_KEYSHR_LOCK)
#define HEAP_XMIN_COMMITTED     0x0100  /* t_xmin committed */
#define HEAP_XMIN_INVALID       0x0200  /* t_xmin invalid/aborted */
#define HEAP_XMIN_FROZEN        (HEAP_XMIN_COMMITTED|HEAP_XMIN_INVALID)
#define HEAP_XMAX_COMMITTED     0x0400  /* t_xmax committed */
#define HEAP_XMAX_INVALID       0x0800  /* t_xmax invalid/aborted */
#define HEAP_XMAX_IS_MULTI      0x1000  /* t_xmax is a MultiXactId */
#define HEAP_UPDATED            0x2000  /* this is UPDATEd version of row */
#define HEAP_MOVED_OFF          0x4000  /* moved to another place by pre-9.0
                                         * VACUUM FULL; kept for binary
                                         * upgrade support */
#define HEAP_MOVED_IN           0x8000  /* moved from another place by pre-9.0
                                         * VACUUM FULL; kept for binary
                                         * upgrade support */
#define HEAP_MOVED (HEAP_MOVED_OFF | HEAP_MOVED_IN)

About t_infomask2:

/*
 * information stored in t_infomask2:
 */
#define HEAP_NATTS_MASK         0x07FF  /* 11 bits for number of attributes */
/* bits 0x1800 are available */
#define HEAP_KEYS_UPDATED       0x2000  /* tuple was updated and key cols
                                         * modified, or tuple deleted */
#define HEAP_HOT_UPDATED        0x4000  /* tuple was HOT-updated */
#define HEAP_ONLY_TUPLE         0x8000  /* this is heap-only tuple */

#define HEAP2_XACT_MASK         0xE000  /* visibility-related bits */

So t_infomask & 256 means that the transaction stored in xmin is marked committed, and t_infomask & 512 means that that the transaction stored in xmin is rolled back or invalid. t_infomask & 1024 and t_infomask & 2048 are the same for xmax.

These are so called “hint bits” that provide a shortcut to commit log information about the visibility of the tuple.

t_infomask2 & 16384 means that the tuple was updated with a HOT update, and t_infomask2 & 32768 means that the tuple is a heap-only tuple.