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Community Blog PostgreSQL 12: B-tree Index Improvements – Duplicate Key and Sort by CTID

PostgreSQL 12: B-tree Index Improvements – Duplicate Key and Sort by CTID

In this article, the author discusses improvements in the B-tree index based on sorting duplicate key values using CTID.

By digoal

Background

PostgreSQL 12 introduces several key enhancements as stated in the official release notes. In particular, we will discuss the improvements in the B-tree index based on sorting duplicate key values using CTID.

Improve performance and space utilization of b-tree indexes with many duplicates (Peter Geoghegan, Heikki Linnakangas). Previously, duplicate index entries were stored unordered within their duplicate groups. This caused overhead during index inserts, wasted space due to excessive page splits, and reduced VACUUM's ability to recycle entire pages. Duplicate index entries are now sorted in heap-storage order.

Indexes pg_upgraded from previous releases will not have these benefits.

Improvements of PG 12 b-tree index storage structure:

1) Duplicate key values are sorted and stored by CTID to reduce the index page splitting. The key values were originally stored in disorder by duplicate groups.

2) During vacuum operation, the table is scanned first. Then, the garbage tuple ctid is stored. After the vacuum memory is full, the indexes are scanned to collect the garbage index tuple. Therefore, when the key values are stored by CTID in order, the vacuum operation generates fewer Write-ahead Logging (WAL) files (fewer index pages involved) if all the duplicate keys are recycled.

Comparison

PG 11

postgres=# create table test_bt (id int, c1 int);  
CREATE TABLE  
postgres=#  create index idx_test_bt_1 on test_bt (c1);  
CREATE INDEX  
postgres=# insert into test_bt select generate_series(1,10000000), random()*10000;           
INSERT 0 10000000  
Time: 44992.592 ms (00:44.993)  
postgres=# \di+ idx_test_bt_1 ;  
                             List of relations  
 Schema |     Name      | Type  |  Owner   |  Table  |  Size  | Description   
--------+---------------+-------+----------+---------+--------+-------------  
 public | idx_test_bt_1 | index | postgres | test_bt | 281 MB |   
(1 row)  
  
  
postgres=# explain (analyze,verbose,timing,costs,buffers) select * from test_bt where c1=2;  
                                                              QUERY PLAN                                                                 
---------------------------------------------------------------------------------------------------------------------------------------  
 Index Scan using idx_test_bt_1 on public.test_bt  (cost=0.38..849.01 rows=50000 width=8) (actual time=0.034..2.011 rows=1062 loops=1)  
   Output: id, c1  
   Index Cond: (test_bt.c1 = 2)  
   Buffers: shared hit=1059  
 Planning Time: 0.132 ms  
 Execution Time: 2.083 ms  
(6 rows)

PG 12

postgres=# create table test_bt (id int, c1 int);  
CREATE TABLE  
postgres=# create index idx_test_bt_1 on test_bt (c1);  
CREATE INDEX  
postgres=# insert into test_bt select generate_series(1,10000000), random()*10000;  
INSERT 0 10000000  
Time: 36449.404 ms (00:36.449)  
  
postgres=# \di+ idx_test_bt_1   
                             List of relations  
 Schema |     Name      | Type  |  Owner   |  Table  |  Size  | Description   
--------+---------------+-------+----------+---------+--------+-------------  
 public | idx_test_bt_1 | index | postgres | test_bt | 235 MB |   
(1 row)  
  
postgres=# explain (analyze,verbose,timing,costs,buffers) select * from test_bt where c1=2;  
                                                               QUERY PLAN                                                                  
-----------------------------------------------------------------------------------------------------------------------------------------  
 Index Scan using idx_test_bt_1 on public.test_bt  (cost=0.43..39395.43 rows=50000 width=8) (actual time=0.033..1.141 rows=1002 loops=1)  
   Output: id, c1  
   Index Cond: (test_bt.c1 = 2)  
   Buffers: shared hit=998  
 Planning Time: 0.074 ms  
 Execution Time: 1.226 ms  
(6 rows)

References

PostgreSQL Database B-tree Index Sort by CTID Duplicate Key
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