Date: Sun, 26 Feb 2023 23:36:18 +0800
From: "jacktby@gmail.com" <jacktby@gmail.com>
To: "Tomas Vondra" <tomas.vondra@enterprisedb.com>
Cc: pgsql-hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: Re: Give me more details of some bits in infomask!!
References: <2023022622305600049813@gmail.com>, 
	<3936d2fb-f229-63fd-26e1-130fbe918f51@enterprisedb.com>
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<html><head><meta http-equiv=3D"content-type" content=3D"text/html; charse=
t=3DUTF-8"><style>body { line-height: 1.5; }blockquote { margin-top: 0px; =
margin-bottom: 0px; margin-left: 0.5em; }body { font-size: 14px; font-fami=
ly: "Microsoft YaHei UI"; color: rgb(0, 0, 0); line-height: 1.5; }</style>=
</head><body>=0A<blockquote style=3D"margin-Top: 0px; margin-Bottom: 0px; =
margin-Left: 0.5em; margin-Right: inherit"><div>&nbsp;</div><div style=3D"=
border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0cm 0cm 0cm"><div=
 style=3D"PADDING-RIGHT: 8px; PADDING-LEFT: 8px; FONT-SIZE: 12px;FONT-FAMI=
LY:tahoma;COLOR:#000000; BACKGROUND: #efefef; PADDING-BOTTOM: 8px; PADDING=
-TOP: 8px"><div><b>From:</b>&nbsp;<a href=3D"mailto:tomas.vondra@enterpris=
edb.com">Tomas Vondra</a></div><div><b>Date:</b>&nbsp;2023-02-26&nbsp;23:2=
3</div><div><b>To:</b>&nbsp;<a href=3D"mailto:jacktby@gmail.com">jacktby@g=
mail.com</a>; <a href=3D"mailto:pgsql-hackers@lists.postgresql.org">pgsql-=
hackers</a></div><div><b>Subject:</b>&nbsp;Re: Give me more details of som=
e bits in infomask!!</div></div></div><div><div>On 2/26/23 15:30, jacktby@=
gmail.com wrote:</div>=0A<div>&gt; here are the source codes from src/incl=
ude/access/htup_details.h.</div>=0A<div>&gt; /*</div>=0A<div>&gt; &nbsp;* =
information stored in t_infomask:</div>=0A<div>&gt; &nbsp;*/</div>=0A<div>=
&gt; #define HEAP_HASNULL0x0001/* has null attribute(s) */</div>=0A<div>&g=
t; #define HEAP_HASVARWIDTH0x0002/* has variable-width attribute(s) */</di=
v>=0A<div>&gt; #define HEAP_HASEXTERNAL0x0004/* has external stored attrib=
ute(s) */</div>=0A<div>&gt; #define HEAP_HASOID_OLD0x0008/* has an object-=
id field */</div>=0A<div>&gt; #define HEAP_XMAX_KEYSHR_LOCK0x0010/* xmax i=
s a key-shared locker */</div>=0A<div>&gt; #define HEAP_COMBOCID0x0020/* t=
_cid is a combo CID */</div>=0A<div>&gt; #define HEAP_XMAX_EXCL_LOCK0x0040=
/* xmax is exclusive locker */</div>=0A<div>&gt; #define HEAP_XMAX_LOCK_ON=
LY0x0080/* xmax, if valid, is only a locker */</div>=0A<div>&gt; </div>=0A=
<div>&gt; And I can't understand these attrs:</div>=0A<div>&nbsp;</div>=0A=
<div>I suggest you try something like 'git grep HEAP_HASEXTERNAL' which sh=
ows</div>=0A<div>you where the flag is used, which should tell you what it=
 means. These</div>=0A<div>short descriptions generally assume you know en=
ough about the internals.</div>=0A<div>&nbsp;</div>=0A<div>&gt; 1.&nbsp;ex=
ternal stored attribute(s), what is this?&nbsp; can you give a create</div=
>=0A<div>&gt; statement to show me?</div>=0A<div>&nbsp;</div>=0A<div>exter=
nal =3D value stored in a TOAST table</div>=0A<div>&nbsp;</div>=0A<div>&gt=
; 2.&nbsp;xmax is a key-shared locker/exclusive locker/only a locker, so h=
ow</div>=0A<div>&gt; you use this? can you give me a&nbsp;&nbsp;scenario?<=
/div>=0A<div>&gt; let me try to explain it:</div>=0A<div>&gt; &nbsp;if the=
re is a txn is trying to read this heaptuple,</div>=0A<div>&gt; the&nbsp;H=
EAP_XMAX_KEYSHR_LOCK bit will be set to 1.</div>=0A<div>&gt; &nbsp;if&nbsp=
;there is a txn is trying to delete/update this heaptuple,</div>=0A<div>&g=
t; the&nbsp;HEAP_XMAX_EXCL_LOCK&nbsp;bit will be set to 1.</div>=0A<div>&g=
t; &nbsp;but for&nbsp;HEAP_XMAX_LOCK_ONLY, I can't understand.</div>=0A<di=
v>&gt; And another thought is that these three bit can have only one to be=
 set</div>=0A<div>&gt; 1 at most.</div>=0A<div>&nbsp;</div>=0A<div>I belie=
ve HEAP_XMAX_LOCK_ONLY means the xmax transaction only locked the</div>=0A=
<div>tuple, without deleting/updating it.</div>=0A<div>&nbsp;</div>=0A<div=
>&gt; 3.&nbsp;t_cid is a combo CID? what's a CID? give me an example pleas=
e.</div>=0A<div>&nbsp;</div>=0A<div>CID means "command ID" i.e. sequential=
 ID assigned to commands in a</div>=0A<div>single session (for visibility =
checks, so that a query doesn't see data</div>=0A<div>deleted by earlier c=
ommands in the same session). See</div>=0A<div>src/backend/utils/time/comb=
ocid.c for basic explanation of what "combo</div>=0A<div>CID" is.</div>=0A=
<div>&nbsp;</div>=0A<div>&nbsp;</div>=0A<div>regards</div>=0A<div>&nbsp;</=
div>=0A<div>-- </div>=0A<div>Tomas Vondra</div>=0A<div>EnterpriseDB: http:=
//www.enterprisedb.com</div>=0A<div>The Enterprise PostgreSQL Company</div=
><div><br></div><div><br></div><div>&gt;&nbsp;<span style=3D"background-co=
lor: transparent;">I believe HEAP_XMAX_LOCK_ONLY means the xmax transactio=
n only locked the</span></div><div>&gt; tuple, without deleting/updating i=
t.</div><div>if so, you mean when I read this tuple, this bit will be set =
1, but I think this is duplicat with&nbsp;<span style=3D"background-color:=
 transparent;">HEAP_XMAX_KEYSHR_LOCK.</span></div><div><span style=3D"back=
ground-color: transparent;"><br></span></div><div><div>&gt; CID means "com=
mand ID" i.e. sequential ID assigned to commands in a</div><div>&gt; singl=
e session (for visibility checks, so that a query doesn't see data</div><d=
iv>&gt; deleted by earlier commands in the same session). See</div><div>&g=
t; src/backend/utils/time/combocid.c for basic explanation of what "combo<=
/div><div>&gt; CID" is.</div></div><div>I think if cid is used for&nbsp;<s=
pan style=3D"background-color: transparent;">&nbsp;</span><span style=3D"b=
ackground-color: transparent;">visibility checks in one session, that's me=
aingless, beacause we can use the t_xmin and t_xmax to&nbsp;</span></div><=
div><span style=3D"background-color: transparent;">get this goal. Is tis&n=
bsp;</span></div>=0A</div></blockquote>=0A</body></html>
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