PostgreSQL 10.0 preview 功能增强 - 唯一约束+附加字段组合功能索引 - 覆盖索引 - covering index

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select * from tbl where c1=? and c2=? and c3=? and c4=?


create index idx on tbl(c1,c2,c3,c4);


create unique index idx on tbl (c1,c2);


PostgreSQL 10.0提供了一个新的功能,可以将这两个索引合并,只有一个索引的体积,同时支持这两个场景。。

create unique index idx on tbl (c1,c2) including (c3,c4);



Hi hackers,  
I'm working on a patch that allows to combine covering and unique   
functionality for btree indexes.  
_Previous discussion was here:_  
1) Proposal thread   
2) Message with proposal clarification   
In a nutshell, the feature allows to create index with "key" columns and   
"included" columns.  
"key" columns can be used as scan keys. Unique constraint relates only   
to "key" columns.  
"included" columns may be used as scan keys if they have suitable opclass.  
Both "key" and "included" columns can be returned from index by   
Btree is the default index and it's used everywhere. So it requires   
properly testing. Volunteers are welcome)  
_Use case:_  
- We have a table (c1, c2, c3, c4);  
- We need to have an unique index on (c1, c2).  
- We would like to have a covering index on all columns to avoid reading   
of heap pages.  
Old way:  
CREATE UNIQUE INDEX olduniqueidx ON oldt USING btree (c1, c2);  
CREATE INDEX oldcoveringidx ON oldt USING btree (c1, c2, c3, c4);  
What's wrong?  
Two indexes contain repeated data. Overhead to data manipulation   
operations and database size.  
New way:  
CREATE UNIQUE INDEX newidx ON newt USING btree (c1, c2) INCLUDING (c3, c4);  
The patch is attached.  
In 'test.sql' you can find a test with detailed comments on each step,   
and comparison of old and new indexes.  
New feature has following syntax:  
CREATE UNIQUE INDEX newidx ON newt USING btree (c1, c2) INCLUDING (c3, c4);  
Keyword INCLUDING defines the "included" columns of index. These columns   
aren't concern to unique constraint.  
Also, them are not stored in index inner pages. It allows to decrease   
index size.  
1) Additional covering index is not required anymore.  
2) New index can use IndexOnlyScan on queries, where old index can't.  
For example,  
explain analyze select c1, c2 from newt where c1<10000 and c3<20;  
*more examples in 'test.sql'  
_Future work:_  
To do opclasses for "included" columns optional.  
CREATE TABLE tbl (c1 int, c4 box);  
If we don't need c4 as an index scankey, we don't need any btree opclass   
on it.  
But we still want to have it in covering index for queries like  
SELECT c4 FROM tbl WHERE c1=1000;  
SELECT * FROM tbl WHERE c1=1000;  
Anastasia Lubennikova  
Postgres Professional:  
The Russian Postgres Company  




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