Postgres是如何管理空值的

创建表test,y字段插入null.

test=# create table test(x bigint,y bigint,z text);
CREATE TABLE
test=# insert into test values(11,null,'abcdefg');
INSERT 0 1
test=# select * from test;
 x  | y |    z
----+---+---------
 11 |   | abcdefg
(1 row)

这条记录存储在数据页里面是:

(gdb) p	*lpp
$17 = {lp_off = 8152, lp_flags = 1, lp_len = 40}

占了40个字节,TupleHeader是24个字节，40-24=16。

我们再插入一条数据看看

test=# insert into test values(22,100,'xyz');
INSERT 0 1
test=# select * from test;
 x  |  y  |    z
----+-----+---------
 11 |     | abcdefg
 22 | 100 | abcdefg
(2 rows)

再看看新插入的记录:

(gdb) p	*lpp
$19 = {lp_off = 8104, lp_flags = 1, lp_len = 48}

这2条数据对比

第一条记录:

insert into test values(11,null,'abcdefg');

字段x是bigint,8字节

字段y是bigint,8字节

字段z是text,字符串strlen('abcdefg')=7,8字节对齐。

8152+40=8192

第二条记录：

insert into test values(22,100,'abcdefg');

8+8+8=24

24+TupleHeader=48字节

8104+48=8152

我们来看看第一条是如何存储NULL的：

代码:src/include/access/tupmacs.h

#define att_isnull(ATT, BITS) (!((BITS)[(ATT) >> 3] & (1 << ((ATT) & 0x07))))

这就是计算字段是否为NULL的宏

PG通过t_bits来标记是否为null.

struct HeapTupleHeaderData
{
	union
	{
		HeapTupleFields t_heap;
		DatumTupleFields t_datum;
	}			t_choice;

	ItemPointerData t_ctid;		/* current TID of this or newer tuple (or a
								 * speculative insertion token) */

	/* Fields below here must match MinimalTupleData! */

	uint16		t_infomask2;	/* number of attributes + various flags */

	uint16		t_infomask;		/* various flag bits, see below */

	uint8		t_hoff;			/* sizeof header incl. bitmap, padding */

	/* ^ - 23 bytes - ^ */

	bits8		t_bits[FLEXIBLE_ARRAY_MEMBER];	/* bitmap of NULLs */

	/* MORE DATA FOLLOWS AT END OF STRUCT */
};

t_bits是一个字节，那么就有8位。所有上面的宏需要根据ATT来移3位。ATT>>3

这是算这个字段是在第几个数组下标

例如我这里查询的是第一个字段1>>3 = 0 就是在第一个数组下标

ATT & 0x07  求字段顺序  第一条是 0 & 0x07 = 0 ,如果是第八个字段也是0，范围就是0-7

test=# select * from test;
 x  |  y  |    z
----+-----+---------
 11 |     | abcdefg
 22 | 100 | abcdefg
(2 rows)

第一条数据t_bits是这样的

0 0 0 0 0 1 0 1

根据上面的宏计算结果:

第一个字段:

(BITS)[(ATT) >> 3] = BITS[0]

(ATT) & 0x07 = 0 & 0x07 = 0

1 << 0 = 1

结果就是

0 0 0 0 0 1 0 1

0 0 0 0 0 0 0 1

!(BITS[0] & 1) = 0

代表数据不算为NULL.

第二个字段:

(BITS)[(ATT) >> 3] = BITS[0]

(ATT) & 0x07 = 1 & 0x07 = 1

1 << 1 = 2

结果就是

0 0 0 0 0 1 0 1

0 0 0 0 0 0 1 0

!(BITS[0] & 2) = 1

代表第二个字段为NULL

我们再插入一条数据

test=# insert into test values(null,null,'abcdefg');
INSERT 0 1
test=# select * from test;
 x  |  y  |    z
----+-----+---------
 11 |     | abcdefg
 22 | 100 | abcdefg
    |     | abcdefg
(3 rows)

我们主要是看新增加的这条数据

(gdb) p	*lpp
$42 = {lp_off = 8072, lp_flags = 1, lp_len = 32}
(gdb)

新插入的数据只占了32个字节 TupleHeader+8

t_bits = 0 0 0 0 0 1 0 0

利用上面的宏也很好的算出前面2个字段为NULL

我们来看看超过8个字段的情况

test=# create table test_more_column(
test(# col1 bigint,
test(# col2 bigint,
test(# col3 bigint,
test(# col4 bigint,
test(# col5 bigint,
test(# col6 bigint,
test(# col7 bigint,
test(# col8 bigint,
test(# col9 bigint,
test(# col10 bigint
test(# );
CREATE TABLE
test=# insert into test_more_column values(1,2,null,4,5,null,7,8,null,10);
INSERT 0 1
test=# select * from test_more_column ;
 col1 | col2 | col3 | col4 | col5 | col6 | col7 | col8 | col9 | col10
------+------+------+------+------+------+------+------+------+-------
    1 |    2 |      |    4 |    5 |      |    7 |    8 |      |    10
(1 row)

test=#

首先看看item

(gdb) p	*lpp
$1 = {lp_off = 8104, lp_flags = 1, lp_len = 88}
(gdb)

总共88个字节

(gdb) p *tuple->t_data
$3 = {t_choice = {t_heap = {t_xmin = 4414, t_xmax = 0, t_field3 = {t_cid = 0, t_xvac = 0}}, t_datum = {datum_len_ = 4414, datum_typmod = 0, datum_typeid = 0}}, t_ctid = {ip_blkid =
 {bi_hi = 0, bi_lo = 0}, ip_posid = 1}, t_infomask2 = 10, t_infomask = 2305, t_hoff = 32 ' ', t_bits = 0x7f8c9af9ef5f "\333\002"}

首先看看头偏移量就改变了不是前面的24个字节。是因为字段超过了8 需要用2个bit来标记NULL，而PG又是8字节对齐所以是24+8=32

88-32=56 总共88字节减去头32  数据占56字节

7 * 8 = 56 上面总共有7个字段存储了值，每个占8字节就是56字节

(gdb) p	bp
$8 = (bits8 *) 0x7f8c9af9ef5f "\333\002"
(gdb) p	sizeof(bp)
$9 = 8
(gdb)

数组的值

(gdb) p	bp[0]
$10 = 219 '\333'
(gdb) p bp[1]
$11 = 2 '\002'

bp[0] = 1 1 0 1 1 0 1 1  = 1 +2 +8 +16 +64 +128 = 219

bp[1] = 0 0 0 0 0 0 1 0  = 2

bp[3] = 0 0 0 0 0 0 0 0 = 0

......

bp[7] = 0 0 0 0 0 0 0 0  = 0

根据上面的宏att_isnull 就能很好的判断出那个字段是NULL。这样就非常的节省了数据存储空间。