《Postgresql 内幕探索》读书笔记 - 第一章：集簇、表空间、元组

select * from pg_database;

postgres=# select * from pg_database;

  oid  |  datname  | datdba | encoding | datcollate  |  datctype   | datistemplate | datallowconn | datconnlimit | datlastsysoid | datfrozenxid | datminmxid | dattablespace |             
  datacl                
-------+-----------+--------+----------+-------------+-------------+---------------+--------------+--------------+---------------+--------------+------------+---------------+-------------
------------------------
 14486 | postgres  |     10 |        6 | en_US.UTF-8 | en_US.UTF-8 | f             | t            |           -1 |         14485 |          727 |          1 |          1663 | 
     1 | template1 |     10 |        6 | en_US.UTF-8 | en_US.UTF-8 | t             | t            |           -1 |         14485 |          727 |          1 |          1663 | {=c/postgres
,postgres=CTc/postgres}
 14485 | template0 |     10 |        6 | en_US.UTF-8 | en_US.UTF-8 | t             | f            |           -1 |         14485 |          727 |          1 |          1663 | {=c/postgres
,postgres=CTc/postgres}
(3 rows)

select relname,oid from pg_class;

postgres=# select relname,oid from pg_class;
                    relname                    |  oid  
-----------------------------------------------+-------
 pg_statistic                                  |  2619
 pg_type                                       |  1247
 pg_toast_1255                                 |  2836
 pg_toast_1255_index                           |  2837
 pg_toast_1247                                 |  4171
 pg_toast_1247_index                           |  4172
 pg_toast_2604                                 |  2830
 pg_toast_2604_index                           |  2831
 pg_toast_2606                                 |  2832
 pg_toast_2606_index                           |  2833
 pg_toast_2612                                 |  4157
 pg_toast_2612_index                           |  4158
 pg_toast_2600                                 |  4159
 pg_toast_2600_index                           |  4160
 pg_toast_2619                                 |  2840
 pg_toast_2619_index                           |  2841
 pg_toast_3381                                 |  3439
 pg_toast_3381_index                           |  3440
 pg_toast_3429                                 |  3430
 pg_toast_3429_index                           |  3431

sudo /usr/pgsql-14/bin/postgresql-14-setup initdb

[root@localhost 14]# pwd
/var/lib/pgsql/14

[root@localhost 14]# ll data/
total 68
drwx------ 5 postgres postgres    41 Jun 22 02:41 base
-rw------- 1 postgres postgres    30 Jun 22 02:41 current_logfiles
drwx------ 2 postgres postgres  4096 Jun 22 02:44 global
drwx------ 2 postgres postgres    32 Jun 22 02:41 log
drwx------ 2 postgres postgres     6 Jun 22 02:41 pg_commit_ts
drwx------ 2 postgres postgres     6 Jun 22 02:41 pg_dynshmem
-rw------- 1 postgres postgres  4577 Jun 22 02:41 pg_hba.conf
-rw------- 1 postgres postgres  1636 Jun 22 02:41 pg_ident.conf
drwx------ 4 postgres postgres    68 Jun 22 02:46 pg_logical
drwx------ 4 postgres postgres    36 Jun 22 02:41 pg_multixact
drwx------ 2 postgres postgres     6 Jun 22 02:41 pg_notify
drwx------ 2 postgres postgres     6 Jun 22 02:41 pg_replslot
drwx------ 2 postgres postgres     6 Jun 22 02:41 pg_serial
drwx------ 2 postgres postgres     6 Jun 22 02:41 pg_snapshots
drwx------ 2 postgres postgres     6 Jun 22 02:41 pg_stat
drwx------ 2 postgres postgres    63 Jun 22 03:30 pg_stat_tmp
drwx------ 2 postgres postgres    18 Jun 22 02:41 pg_subtrans
drwx------ 2 postgres postgres     6 Jun 22 02:41 pg_tblspc
drwx------ 2 postgres postgres     6 Jun 22 02:41 pg_twophase
-rw------- 1 postgres postgres     3 Jun 22 02:41 PG_VERSION
drwx------ 3 postgres postgres    60 Jun 22 02:41 pg_wal
drwx------ 2 postgres postgres    18 Jun 22 02:41 pg_xact
-rw------- 1 postgres postgres    88 Jun 22 02:41 postgresql.auto.conf
-rw------- 1 postgres postgres 28776 Jun 22 02:41 postgresql.conf
-rw------- 1 postgres postgres    58 Jun 22 02:41 postmaster.opts
-rw------- 1 postgres postgres   103 Jun 22 02:41 postmaster.pid

create table db_test(id int primary key, name varchar(50), age int);

postgres=# select * from pg_class where relname ='db_test';
  oid  | relname | relnamespace | reltype | reloftype | relowner | relam | relfilenode | reltablespace | relpages | reltuples | relallvisible | reltoastrelid | relhasindex | relisshared | relpersistence
 | relkind | relnatts | relchecks | relhasrules | relhastriggers | relhassubclass | relrowsecurity | relforcerowsecurity | relispopulated | relreplident | relispartition | relrewrite | relfrozenxid | re
lminmxid | relacl | reloptions | relpartbound 
-------+---------+--------------+---------+-----------+----------+-------+-------------+---------------+----------+-----------+---------------+---------------+-------------+-------------+---------------
-+---------+----------+-----------+-------------+----------------+----------------+----------------+---------------------+----------------+--------------+----------------+------------+--------------+---
---------+--------+------------+--------------
 16384 | db_test |         2200 |   16386 |         0 |       10 |     2 |       16384 |             0 |        0 |        -1 |             0 |             0 | t           | f           | p             
 | r       |        3 |         0 | f           | f              | f              | f              | f                   | t              | d            | f              |          0 |          734 |   
       1 |        |            | 
(1 row)

postgres=# truncate db_test;
TRUNCATE TABLE

postgres=# select * from pg_class where relname ='db_test';
  oid  | relname | relnamespace | reltype | reloftype | relowner | relam | relfilenode | reltablespace | relpages | reltuples | relallvisible | reltoastrelid | relhasindex | relisshared | relpersistence
 | relkind | relnatts | relchecks | relhasrules | relhastriggers | relhassubclass | relrowsecurity | relforcerowsecurity | relispopulated | relreplident | relispartition | relrewrite | relfrozenxid | re
lminmxid | relacl | reloptions | relpartbound 
-------+---------+--------------+---------+-----------+----------+-------+-------------+---------------+----------+-----------+---------------+---------------+-------------+-------------+---------------
-+---------+----------+-----------+-------------+----------------+----------------+----------------+---------------------+----------------+--------------+----------------+------------+--------------+---
---------+--------+------------+--------------
 16384 | db_test |         2200 |   16386 |         0 |       10 |     2 |       16389 |             0 |        0 |        -1 |             0 |             0 | t           | f           | p             
 | r       |        3 |         0 | f           | f              | f              | f              | f                   | t              | d            | f              |          0 |          735 |   
       1 |        |            | 
(1 row)

 select oid,relname,relfilenode from pg_class limit 10;

	 oid  |       relname       | relfilenode 
	------+---------------------+-------------
	 2619 | pg_statistic        |        2619
	 1247 | pg_type             |           0
	 2836 | pg_toast_1255       |           0
	 2837 | pg_toast_1255_index |           0
	 4171 | pg_toast_1247       |           0
	 4172 | pg_toast_1247_index |           0
	 2830 | pg_toast_2604       |        2830
	 2831 | pg_toast_2604_index |        2831
	 2832 | pg_toast_2606       |        2832
	 2833 | pg_toast_2606_index |        2833

select pg_relation_filepath('pg_statistic') from pg_class limit 20;

	 pg_relation_filepath 
	----------------------
	 base/14486/2619
	 base/14486/2619
	 base/14486/2619
	 base/14486/2619
	 base/14486/2619
	 base/14486/2619
	 base/14486/2619

CREATE SEQUENCE upms_log_id_seq START 10;

postgres=# CREATE SEQUENCE upms_log_id_seq START 10;
CREATE SEQUENCE

CREATE TABLE "public"."t_user" (
	"id" int8 NOT NULL DEFAULT nextval( 'upms_log_id_seq' :: regclass ),
	"name" VARCHAR ( 255 ) COLLATE "pg_catalog"."default",
	"phone" VARCHAR ( 255 ) COLLATE "pg_catalog"."default",
	"birthday" TIMESTAMP ( 6 ),
	"sex" VARCHAR ( 64 ) COLLATE "pg_catalog"."default",
	CONSTRAINT "t_user_pkey" PRIMARY KEY ( "id" ) 
);

ALTER TABLE "public"."t_user" OWNER TO "postgres";

select substr( 'abcdefghijklmnopqrstuvwxyz', 1, ( random( ) * 26 ) :: INTEGER )；

SELECT lpad( ( random( ) * 138 ) :: INT :: TEXT, 11, '0' );

SELECT date(generate_series(now(), now() + '1 week', '1 day'));

SELECT lpad( ( random( ) * 1 ) :: INT :: text, 1, '0' );

EXPLAIN ANALYZE INSERT INTO t_user 
SELECT  generate_series ( 1, 10000000 ),
substr( 'abcdefghijklmnopqrstuvwxyz', 1, ( random( ) * 26 ) :: INTEGER ),
lpad( ( random( ) * 138 ) :: INT :: TEXT, 11, '0' ),
DATE ( generate_series ( now( ), now( ) + '1 week', '1 day' ) ),
lpad( ( random( ) * 1 ) :: INT :: TEXT, 1, '0' );

postgres=# EXPLAIN ANALYZE INSERT INTO t_user 
postgres-# SELECT  generate_series ( 1, 10000000 ),
postgres-# substr( 'abcdefghijklmnopqrstuvwxyz', 1, ( random( ) * 26 ) :: INTEGER ),
postgres-# lpad( ( random( ) * 138 ) :: INT :: TEXT, 11, '0' ),
postgres-# DATE ( generate_series ( now( ), now( ) + '1 week', '1 day' ) ),
postgres-# lpad( ( random( ) * 1 ) :: INT :: TEXT, 1, '0' );
                                                               QUERY PLAN                                                                
-----------------------------------------------------------------------------------------------------------------------------------------
 Insert on t_user  (cost=0.00..925000.03 rows=0 width=0) (actual time=53327.150..53327.162 rows=0 loops=1)
   ->  Subquery Scan on "*SELECT*"  (cost=0.00..925000.03 rows=10000000 width=1194) (actual time=0.067..19638.272 rows=10000000 loops=1)
         ->  Result  (cost=0.00..700000.03 rows=10000000 width=104) (actual time=0.059..16112.101 rows=10000000 loops=1)
               ->  ProjectSet  (cost=0.00..50000.03 rows=10000000 width=12) (actual time=0.017..2277.500 rows=10000000 loops=1)
                     ->  Result  (cost=0.00..0.01 rows=1 width=0) (actual time=0.003..0.010 rows=1 loops=1)
 Planning Time: 1.396 ms
 Execution Time: 53335.404 ms
(7 rows)

postgres=# select pg_relation_filepath('t_user');
 pg_relation_filepath 
----------------------
 base/14486/16398
(1 row)

[root@localhost 14486]# pwd
/var/lib/pgsql/14/data/base/14486


-rw------- 1 postgres postgres 1073741824 Jun 22 17:35 16398
-rw------- 1 postgres postgres  282771456 Jun 22 17:36 16398.1
-rw------- 1 postgres postgres     352256 Jun 22 17:35 16398_fsm
-rw------- 1 postgres postgres      24576 Jun 22 17:35 16398_vm

[root@localhost 14486]# pwd
/var/lib/pgsql/14/data/base/14486


-rw------- 1 postgres postgres 1073741824 Jun 22 17:35 16398
-rw------- 1 postgres postgres  282771456 Jun 22 17:36 16398.1
-rw------- 1 postgres postgres     352256 Jun 22 17:35 16398_fsm
-rw------- 1 postgres postgres      24576 Jun 22 17:35 16398_vm

PG_主版本号_目录版本号

postgres=# create tablespace tbs_test owner postgres location '/opt/postgres/tbs_test';
ERROR:  directory "/opt/postgres/tbs_test" does not exist

postgres=# create tablespace tbs_test owner postgres location '/opt/postgres/tbs_test';
ERROR:  could not set permissions on directory "/opt/postgres/tbs_test": Operation not permitted

[root@localhost 14486]# mkdir -p /opt/postgres/tbs_test

[root@localhost 14486]# chown postgres:postgres /opt/postgres/
[root@localhost 14486]# ll /opt
total 0
drwxr-xr-x 3 postgres postgres 22 Jun 22 18:20 postgres

postgres=# create tablespace tbs_test owner postgres location '/opt/postgres/tbs_test';
CREATE TABLESPACE

[root@localhost data]# pwd 
/var/lib/pgsql/14/data
[root@localhost data]# ll
total 72
......

drwx------ 2 postgres postgres    19 Jun 22 18:22 pg_tblspc

.....

[root@localhost data]# tree pg_tblspc/
pg_tblspc/
└── 16408 -> /opt/postgres/tbs_test

[root@localhost data]# ll /opt/postgres/tbs_test
total 0
drwx------ 2 postgres postgres 6 Jun 22 18:22 PG_14_202107181

postgres=# create table newtab(id int) tablespace tbs_test;
CREATE TABLE

postgres=# select pg_relation_filepath('newtab');

pg_relation_filepath             
---------------------------------------------
 pg_tblspc/16408/PG_14_202107181/14486/16409
(1 row)

postgres=# select relname,oid from pg_class where relname='newtab';
 relname |  oid  
---------+-------
 newtab  | 16409
(1 row)

ERROR:  tablespace "tbs_test" is not empty

postgres=# drop table if exists newtab;
DROP TABLE

postgres=# drop tablespace if exists tbs_test;
DROP TABLESPACE

[root@localhost data]# ll /opt/postgres/tbs_test/
total 0

typedef struct PageHeaderData
{
	//  XXX LSN是任何块的成员，不仅是页面组织的成员。
	/* XXX LSN is member of *any* block, not only page-organized ones */
	
	//本页面最近变更对应xlog记录的标识
	// 用于记录该页的最后一次变化 
	PageXLogRecPtr pd_lsn;		/* LSN: next byte after last byte of xlog
								 * record for last change to this page */
	// 校验和
	uint16		pd_checksum;	/* checksum */
	
	// 标记位
	uint16		pd_flags;		/* flag bits, see below */
	
	// 空闲空间起始位置
	LocationIndex pd_lower;		/* offset to start of free space */
	
	// 空闲空间终止位置
	LocationIndex pd_upper;		/* offset to end of free space */

	// 特殊用途空间的开始位置
	LocationIndex pd_special;	/* offset to start of special space */

	// 页面版本编号（尺寸必须为256的倍数，留下低8位用于页面版本编号）
	uint16		pd_pagesize_version;
	
	// 最老的可修剪XID, 如果没有设置为0
	TransactionId pd_prune_xid; /* oldest prunable XID, or zero if none */

	// 行指针数组
	ItemIdData	pd_linp[FLEXIBLE_ARRAY_MEMBER]; /* line pointer array */
} PageHeaderData;

typedef PageHeaderData *PageHeader;

// 缓冲区页中的项目指针(item pointer)，也被称为行指针(line pointer)
typedef struct ItemIdData 	ItemIdData

// 元组偏移量 (相对页面起始处)
unsigned 	lp_off:15

// 行指针的状态
unsigned 	lp_flags:2

// 元组的字节长度
// lp_len == 0 表示该行指针没有关联存储。独立于其lp_flags的状态
unsigned 	lp_len:15

#define LP_UNUSED       0       // 空闲行指针 (lp_len必需始终为0)
#define LP_NORMAL       1       // 行指针被使用，指向一个元组 (lp_len必需始终>0)
#define LP_REDIRECT     2       // HOT技术标识 (lp_len必需为0)
#define LP_DEAD         3       // 行指针对应的元组为死元组

 /*
  * mask_lp_flags
  *
  * In some index AMs, line pointer flags can be modified on the primary
  * without emitting any WAL record.
  */
 void
 mask_lp_flags(Page page)
 {
     OffsetNumber offnum,
                 maxoff;
	 // 获取页面最大偏移量
     maxoff = PageGetMaxOffsetNumber(page);
     // 交易当前的偏移量是否合法
     for (offnum = FirstOffsetNumber;
          offnum <= maxoff;
          offnum = OffsetNumberNext(offnum))
     {
	     // 根据偏移量取出对应的编号
         ItemId      itemId = PageGetItemId(page, offnum);
         
		// 检查当前的编号是否被使用，根据结果更新lp_flags标记。
         if (ItemIdIsUsed(itemId))
             itemId->lp_flags = LP_UNUSED;
     }
 }

 /*
  * A line pointer on a buffer page.  See buffer page definitions and comments
  * for an explanation of how line pointers are used.
  * 缓冲区页面上的一个行指针。关于行指针的使用方法，请参见缓冲区页面的定义和注释。
  * 
  * In some cases a line pointer is "in use" but does not have any associated
  * storage on the page.  By convention, lp_len == 0 in every line pointer
  * that does not have storage, independently of its lp_flags state.
  * 在某些情况下，行指针是 "使用中"z状态，但在页面上没有任何相关的存储。
  * 根据惯例，在每一个没有存储空间的行指针中，lp_len == 0。都没有存储空间，这与它的lp_flags状态无关。
  */
 typedef struct ItemIdData
 {
     unsigned    lp_off:15,      /* offset to tuple (from start of page)从页面的开始到元组的偏移量*/
                 lp_flags:2,     /* state of line pointer, see below 行指针状态 */
                 lp_len:15;      /* byte length of tuple 元组的字节长度  */
 } ItemIdData;
  
 typedef ItemIdData *ItemId;
  
 /*
  * lp_flags has these possible states.  An UNUSED line pointer is available
  * for immediate re-use, the other states are not.
  */
 #define LP_UNUSED       0       /* unused (should always have lp_len=0) */
 #define LP_NORMAL       1       /* used (should always have lp_len>0) */
 #define LP_REDIRECT     2       /* HOT redirect (should have lp_len=0) */
 #define LP_DEAD         3       /* dead, may or may not have storage */

/* 负责删除指定位置的数据，删除数据后，会将需要将空闲的数据指针和数据进行压缩合并 */
void PageIndexTupleDelete(Page page, OffsetNumber  offnum ) 
{
     PageHeader  phdr = (PageHeader) page;
     char       *addr;
     ItemId      tup;
     Size        size;
     unsigned    offset;
     int         nbytes;
     int         offidx;
     int         nline;
  
     /*
      * As with PageRepairFragmentation, paranoia seems justified.
      */
     if (phdr->pd_lower < SizeOfPageHeaderData ||
         phdr->pd_lower > phdr->pd_upper ||
         phdr->pd_upper > phdr->pd_special ||
         phdr->pd_special > BLCKSZ ||
         phdr->pd_special != MAXALIGN(phdr->pd_special))
         ereport(ERROR,
                 (errcode(ERRCODE_DATA_CORRUPTED),
                  errmsg("corrupted page pointers: lower = %u, upper = %u, special = %u",
                         phdr->pd_lower, phdr->pd_upper, phdr->pd_special)));
  
     nline = PageGetMaxOffsetNumber(page);
     if ((int) offnum <= 0 || (int) offnum > nline)
         elog(ERROR, "invalid index offnum: %u", offnum);
  
     /* change offset number to offset index */
     offidx = offnum - 1;
  
     tup = PageGetItemId(page, offnum);
     Assert(ItemIdHasStorage(tup));
     size = ItemIdGetLength(tup);
     offset = ItemIdGetOffset(tup);
  
     if (offset < phdr->pd_upper || (offset + size) > phdr->pd_special ||
         offset != MAXALIGN(offset))
         ereport(ERROR,
                 (errcode(ERRCODE_DATA_CORRUPTED),
                  errmsg("corrupted line pointer: offset = %u, size = %u",
                         offset, (unsigned int) size)));
   
     /* Amount of space to actually be deleted */
     size = MAXALIGN(size);
   // 首先删除行指针，然后把后面的位置向前填充补齐空位，如果删除**pd_lower**指向位置则不需要移动，只需要对应更新为上一个行指针即可。
     /*
      * First, we want to get rid of the pd_linp entry for the index tuple. We
      * copy all subsequent linp's back one slot in the array. We don't use
      * PageGetItemId, because we are manipulating the _array_, not individual
      * linp's.
      */
     nbytes = phdr->pd_lower -
         ((char *) &phdr->pd_linp[offidx + 1] - (char *) phdr);
  
     if (nbytes > 0)
         memmove((char *) &(phdr->pd_linp[offidx]),
                 (char *) &(phdr->pd_linp[offidx + 1]),
                 nbytes);
   // 删除实际的堆元组。对应的也需要进行填补空缺，如果删除**pd_upper**指向位置则不需要移动，只需要更新为后一个即可
     /*
      * Now move everything between the old upper bound (beginning of tuple
      * space) and the beginning of the deleted tuple forward, so that space in
      * the middle of the page is left free.  If we've just deleted the tuple
      * at the beginning of tuple space, then there's no need to do the copy.
      */
  
     /* beginning of tuple space */
     addr = (char *) page + phdr->pd_upper;
  
     if (offset > phdr->pd_upper)
         memmove(addr + size, addr, offset - phdr->pd_upper);
  
     /* adjust free space boundary pointers */
     phdr->pd_upper += size;
     phdr->pd_lower -= sizeof(ItemIdData);


  // 3.  数据的存储位置发生移动，更新数据指针的 offset 属性。
     /*
      * Finally, we need to adjust the linp entries that remain.
      *
      * Anything that used to be before the deleted tuple's data was moved
      * forward by the size of the deleted tuple.
      */
     if (!PageIsEmpty(page))
     {
         int         i;
  
         nline--;                /* there's one less than when we started */
         for (i = 1; i <= nline; i++)
         {
             ItemId      ii = PageGetItemId(page, i);
  
             Assert(ItemIdHasStorage(ii));
             if (ItemIdGetOffset(ii) <= offset)
                 ii->lp_off += size;
         }
     }
 }

bool PageIndexTupleOverwrite(Page page, OffsetNumber offnum, Item newtup, Size newsize);

 {
     PageHeader  phdr = (PageHeader) page;
     ItemId      tupid;
     int         oldsize;
     unsigned    offset;
     Size        alignednewsize;
     int         size_diff;
     int         itemcount;
  
     /*
      * As with PageRepairFragmentation, paranoia seems justified.
      */
     if (phdr->pd_lower < SizeOfPageHeaderData ||
         phdr->pd_lower > phdr->pd_upper ||
         phdr->pd_upper > phdr->pd_special ||
         phdr->pd_special > BLCKSZ ||
         phdr->pd_special != MAXALIGN(phdr->pd_special))
         ereport(ERROR,
                 (errcode(ERRCODE_DATA_CORRUPTED),
                  errmsg("corrupted page pointers: lower = %u, upper = %u, special = %u",
                         phdr->pd_lower, phdr->pd_upper, phdr->pd_special)));
  
     itemcount = PageGetMaxOffsetNumber(page);
     if ((int) offnum <= 0 || (int) offnum > itemcount)
         elog(ERROR, "invalid index offnum: %u", offnum);
  
     tupid = PageGetItemId(page, offnum);
     Assert(ItemIdHasStorage(tupid));
     oldsize = ItemIdGetLength(tupid);
     offset = ItemIdGetOffset(tupid);
  
     if (offset < phdr->pd_upper || (offset + oldsize) > phdr->pd_special ||
         offset != MAXALIGN(offset))
         ereport(ERROR,
                 (errcode(ERRCODE_DATA_CORRUPTED),
                  errmsg("corrupted line pointer: offset = %u, size = %u",
                         offset, (unsigned int) oldsize)));
  
     /*
      * Determine actual change in space requirement, check for page overflow.
      */
     oldsize = MAXALIGN(oldsize);
     alignednewsize = MAXALIGN(newsize);
     if (alignednewsize > oldsize + (phdr->pd_upper - phdr->pd_lower))
         return false;

	// 重新定位现有数据并更新行指针，除非新的元组与旧元组的大小相同（对齐后）,要重新定位的是目标元组之前的数据
     /*
      * Relocate existing data and update line pointers, unless the new tuple
      * is the same size as the old (after alignment), in which case there's
      * nothing to do.  Notice that what we have to relocate is data before the
      * target tuple, not data after, so it's convenient to express size_diff
      * as the amount by which the tuple's size is decreasing, making it the
      * delta to add to pd_upper and affected line pointers.
      */
     size_diff = oldsize - (int) alignednewsize;
     if (size_diff != 0)
     {
         char       *addr = (char *) page + phdr->pd_upper;
         int         i;
  
         /* relocate all tuple data before the target tuple */
         memmove(addr + size_diff, addr, offset - phdr->pd_upper);
  
         /* adjust free space boundary pointer */
         phdr->pd_upper += size_diff;
  
         /* adjust affected line pointers too */
         for (i = FirstOffsetNumber; i <= itemcount; i++)
         {
             ItemId      ii = PageGetItemId(page, i);
  
             /* Allow items without storage; currently only BRIN needs that */
             if (ItemIdHasStorage(ii) && ItemIdGetOffset(ii) <= offset)
                 ii->lp_off += size_diff;
         }
     }
  
     /* Update the item's tuple length without changing its lp_flags field */
     tupid->lp_off = offset + size_diff;
     tupid->lp_len = newsize;
  
     /* Copy new tuple data onto page */
     memcpy(PageGetItem(page, tupid), newtup, newsize);
  
     return true;
 }

sampledb=# SELECT ctid, data FROM sampletbl WHERE ctid = '(0,1)';
 ctid  |   data    
-------+-----------
 (0,1) | AAAAAAAAA
(1 row)

sampledb=# EXPLAIN SELECT ctid, data FROM sampletbl WHERE ctid = '(0,1)';
                        QUERY PLAN
----------------------------------------------------------
 Tid Scan on sampletbl  (cost=0.00..1.11 rows=1 width=38)
   TID Cond: (ctid = '(0,1)'::tid)

int compare(Datum a, Datum b)

Datum * extractValue(Datum itemValue, int32 * nkeys, bool ** nullFlags)

** pmatch, Pointer ** extra_data, bool ** nullFlags, int32 * searchMode)