Question

I have a table (let’s called it table_a). It got around 15 million rows. It’s a simple table that have a primary key.

Recently I created a backup table(let’s called it table_a_bkp) and moved 12 million rows from table_a.
I used simple SQL (delete/insert) to perform the task.
Both tables have same structure and use same tablespace.

Query speed of table_a doesn’t improved even total data rows reduced to 2+ million.
In fact table_a_bkp(12m rows) even have faster query speed than table_a(2m rows).

Checked with pg_stat_all_tables two tables both seems auto vacuum & analyze after deletion performed.

I expected table_a query speed should be improved as it got much less data now…

DB Version : PostgreSQL 9.1 hosted on Linux

EXPLAIN (backup table is faster than 1st table even rows is much larger) :

EXPLAIN (ANALYZE TRUE, COSTS TRUE, BUFFERS TRUE) select count(*) from txngeneral
"Aggregate  (cost=742732.94..742732.95 rows=1 width=0) (actual time=73232.598..73232.599 rows=1 loops=1)"
"  Buffers: shared hit=8910 read=701646"
"  ->  Seq Scan on txngeneral  (cost=0.00..736297.55 rows=2574155 width=0) (actual time=17.614..72763.873 rows=2572550 loops=1)"
"        Buffers: shared hit=8910 read=701646"
"Total runtime: 73232.647 ms"



EXPLAIN (ANALYZE TRUE, COSTS TRUE, BUFFERS TRUE) select count(*) from txngeneral_bkp
"Aggregate  (cost=723840.13..723840.14 rows=1 width=0) (actual time=57134.270..57134.270 rows=1 loops=1)"
"  Buffers: shared hit=96 read=569895"
"  ->  Seq Scan on txngeneral_bkp  (cost=0.00..693070.30 rows=12307930 width=0) (actual time=5.436..54889.543 rows=12339180 loops=1)"
"        Buffers: shared hit=96 read=569895"
"Total runtime: 57134.321 ms"

Resolved: VACUUM FULL did speed up table scan.

Answer 1

You should VACUUM ANALYZE your original table. (Probably a full VACUUM will be needed.) Since the new table was created and populated all at once, they form more or less contiguous ‘blocks’, while the tuples of original table are spread all over the disk.