Performance improvements in PostgreSQL 9.5 and beyond

Performance improvements in
PostgreSQL 9.5 (and beyond)
pgconf.eu 2015, October 27-30, Vienna
Tomas Vondra
tomas.vondra@2ndquadrant.com

PostgreSQL 9.5, 9.6, ...
● many improvements
– many of them related to performance
– many quite large
● release notes are good overview, but ...
– many changes not mentioned explicitly
– often difficult to get an idea of the impact
● many talks about new features in general
– this talk is about changes affecting performance

What we'll look at?
● PostgreSQL 9.5
● PostgreSQL 9.6+
– committed
– still being worked on (commitfests)
● only “main” improvements
– complete “features” (multiple commits)
– try to showcase them, show the impact
– no particular order
● won't mention too many low-level optimizations

slides
http://www.slideshare.net/fuzzycz/performance-in-pg95
test scripts
https://github.com/2ndQuadrant/performance-in-pg95

Sorting
● allow sorting by inlined, non-SQL-callable functions
– reduces per-call overhead
● use abbreviated keys for faster sorting
– VARCHAR, TEXT, NUMERIC
– Does not apply to CHAR values!
● stuff using “Sort Support” benefits from this
– CREATE INDEX, REINDEX, CLUSTER
– ORDER BY (when not executed using an index)

Sorting
CREATE TABLE test_text_random AS
SELECT md5(i::text) AS val
FROM generate_series(1, 50.000.000) s(i);
CREATE TABLE test_text_asc AS
SELECT * from test_text_random
ORDER BY 1;
SELECT COUNT(1) FROM (
SELECT * FROM test_text_random ORDER BY 1
) foo;

asc desc almost asc almost desc random
0
50
100
150
200
250
300
350
400
450
500
Sorting improvements in PostgreSQL 9.5
sort duration on 50M rows (TEXT)
PostgreSQL 9.4 PostgreSQL 9.5
dataset type
duration[seconds]

0
50
100
150
200
250
300
350
Sorting improvements in PostgreSQL 9.5
sort duration on 50M rows (TEXT)
PostgreSQL 9.4 PostgreSQL 9.5
dataset type
duration[seconds]

0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
Sorting speedups on PostgreSQL 9.5
speedup on 50M rows (TEXT and NUMERIC)
text numeric
dataset type
relativespeedup

Hash Joins
● reduce palloc overhead
– dense packing of tuples (trivial local allocator, same life-span)
– significant reduction of overhead (both space and time)
● reduce NTUP_PER_BUCKET to 1 (from 10)
– goal is less that 1 tuple per bucket (on average)
– significant speedup of lookups
● dynamically resize the hash table
– handle under-estimates gracefully
– otherwise easily 100s of tuples per bucket (linked list)

Hash Joins
CREATE TABLE test_dim AS
SELECT (i-1) AS id, md5(i::text) AS val
FROM generate_series(1,100.000) s(i);
CREATE TABLE test_fact AS
SELECT mod(i,100.000) AS dim_id, md5(i::text) AS val
FROM generate_series(1,50.000.000) s(i);
SELECT count(*) FROM test_fact
JOIN test_dim ON (dim_id = id);

0 500000 1000000 1500000 2000000
0
10000
20000
30000
40000
50000
PostgreSQL 9.5 Hash Join Improvements
join duration - 50M rows (outer), different NTUP_PER_BUCKET
NTUP_PER_BUCKET=10 NTUP_PER_BUCKET=1
hash size (number of tuples)
duration[miliseconds]

0 500000 1000000 1500000 2000000
0
10000
20000
30000
40000
50000
PostgreSQL 9.5 Hash Join Improvements
join duration - 50M rows (outer), different NTUP_PER_BUCKET
NTUP_PER_BUCKET=10 NTUP_PER_BUCKET=1
PostgreSQL 9.5
hash size (number of inner tuples)

Indexes
● CREATE INDEX
– avoid copying index tuples when building an index (palloc overhead)
● Index-only scans with GiST
– support to range type, inet GiST opclass and btree_gist
● Bitmap Index Scan
– in some cases up to 50% was spent in tbm_add_tuples
– cache the last accessed page in tbm_add_tuples
● BRIN
– block range indexes, tracking min/max per block
– only bitmap index scans (equality and range queries)

Bitmap build speedup
CREATE EXTENSION btree_gin;
CREATE TABLE t AS
SELECT (v / 10)::int4 AS i
FROM generate_series(1, 5.000.000) AS v;
CREATE INDEX idx ON t USING gin (i);
SET enable_seqscan = off;
SELECT * FROM t WHERE i >= 0;
SELECT * FROM t WHERE i >= 100 AND i<= 100;

query 1 query 2
0
500
1000
1500
2000
2500
3000
Bitmap build speedup
cache last page in tbm_add_tuples()
before after

BRIN Indexes
-- data preparation
CREATE TABLE test_bitmap AS
SELECT mod(i, 100.000) AS val
CREATE INDEX test_btree_idx ON test_bitmap(val);
CREATE INDEX test_brin_idx ON test_bitmap USING brin(val);
-- benchmark
SET enable_seqscan = off;
SET enable_indexscan = off;
SELECT COUNT(*) FROM test_bitmap WHERE val <= $1;

0.00% 20.00% 40.00% 60.00% 80.00% 100.00%
0
5000
10000
15000
20000
BRIN vs. BTREE
Bitmap Index Scan on 100M rows (sorted)
BTREE BRIN (128) BRIN (4)
fraction of table matching the condition
duration

btree BRIN (1) BRIN (4) BRIN (128)
0
500
1000
1500
2000
2500
2142
11 2.8 0.13
BRIN vs. BTREE
Index size on 100M rows (sorted)
size(MB)

Aggregate functions
● Use 128-bit math to accelerate some aggregation functions.
– some INT aggregate functions used NUMERIC for internal state
– requires support for 128-bit integers (if provided by compiler).
● impacted aggregates
– sum(int8)
– avg(int8)
– var_*(int2)
– var_*(int4)
– stdev_*(int2)
– stdev_*(int4)

Aggregate functions
CREATE TABLE test_aggregates AS
SELECT i AS a, i AS b
SELECT SUM(a), AVG(b) FROM test_aggregates;

before after
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
20000
16834
5226
Aggregate functions / 128-bit state
using 128-bit integers for state (instead of NUMERIC)

PL/pgSQL
● Support "expanded" objects, particularly arrays, for better
performance.
● Allocate ParamListInfo once per plpgsql function, not once per
expression.
● Use standard casting mechanism to convert types in plpgsql,
when possible.
● Use fast path in plpgsql's RETURN/RETURN NEXT in more
cases.

Planner and optimizer
● remove unnecessary references to left outer join subqueries
● pushdown of query restrictions into window functions
● simplification of EXISTS() subqueries containing LIMIT
● teach predtest.c that "foo" implies "foo IS NOT NULL"
● improve predtest.c's ability to reason about operator
expressions

Locking and concurrency
● checksum improvements
– Speed up CRC calculation using slicing-by-8 algorithm.
– Use Intel SSE 4.2 CRC instructions where available.
– Optimize pg_comp_crc32c_sse42 routine slightly, and
also use it on x86.
● add a basic atomic ops API abstracting away
platform/architecture details.
● reduce lock levels of some trigger DDL and add FKs

Locking and concurrency
● Improve LWLock scalability.
● various shared buffer improvements
– Improve concurrency of shared buffer replacement
– Increase the number of buffer mapping partitions to 128.
– Lockless StrategyGetBuffer clock sweep hot path.
– Align buffer descriptors to cache line boundaries.
– Make backend local tracking of buffer pins memory efficient
– Reduce the number of page locks and pins during index scans
– Optimize locking a tuple already locked by another subxact

Parallel Seq Scan
SET max_parallel_degree = 4;
SELECT COUNT(*) FROM test_parallel WHERE test_func(a, 1);
QUERY PLAN
------------------------------------------------------------
Aggregate (cost=15411721.93..15411721.94 rows=1 width=0)
-> Gather (cost=1000.00..15328388.60 rows=33333330 width=0)
Number of Workers: 4
-> Partial Seq Scan on test_parallel
(cost=0.00..5327388.60 rows=33333330 width=0)
Filter: test_func(a, 1)

0 1 2 3 4 5
0
20
40
60
80
100
120
140
Parallel Seq Scan
speedup for selectivity and parallel degree (100M rows)
1.00% 25.00% 90.00% 100.00%
max_parallel_degree
duration[seconds]

TABLESAMPLE
SELECT * FROM t TABLESAMPLE sampling_method (args)
[REPEATABLE (seed)]
SELECT * FROM t TABLESAMPLE BERNOULLI (33.3);
SELECT * FROM t TABLESAMPLE SYSTEM (33.3);
-- tsm_system_rows
SELECT * FROM t TABLESAMPLE SYSTEM_ROWS (1000);
-- tsm_system_time
SELECT * FROM t TABLESAMPLE SYSTEM_TIME (1000);

0 10 20 30 40 50 60 70 80 90 100
0
5000
10000
15000
20000
25000
30000
TABLESAMPLE
sampling duration
seq scan bernoulli system
sample size (percent of tuples)

Aggregate functions
● some aggregates use the same state
– AVG, SUM, …
– we’re keeping it separate and updating it twice
– but only the final function is actually different
● so …
Share transition state between different
aggregates when possible.

Aggregate functions
CREATE TABLE test_aggregates AS
SELECT i AS a
SELECT SUM(a), AVG(a) FROM test_aggregates;

BIGINT NUMERIC
0
2000
4000
6000
8000
10000
12000
14000
5438
12858
4056
8103
Aggregate functions
sharing aggregate state
before after

Disabling HOT cleanup
● HOT allows UPDATEs without bloating indexes
– a page may have and many “previous” tuple versions
– the dead versions are cleaned by VACUUM or by
queries reading the block
– single query may be forced to cleanup the whole table
(e.g. after a batch update)
– clear impact on performance, a bit unpredictable
● the patch attempts to somehow limit the impact
– query only fixes limited number of pages, etc.

Checkpoints
● continuous flushing (and sorting writes)
– more about variance than about throughput
– eliminate latency stalls / spikes due to checkpoints
– effect depends on I/O scheduler, storage, ...
● compensate for full_page_writes
– spread checkpoints assume constant WAL rate
– not really true due to initial rush to write full pages
– scheduling gets confused by this difference
– patch tries to compensate for this effect

Freezing large tables
● every time we “run out of XIDs” we need to freeze tuples
– we have to scan all the tables to freeze all pages
– even if many of the pages are already “fully frozen”
– serious problem on large databases
– users often postpone the freezing (and then DB shuts down)
● add “all tuples frozen” into visibility map
– allows skipping already frozen pages
● patch seems mostly ready
– mostly discussions about renaming (vm or vfm?)

Additional 9.6+ changes
● Locking and concurrency
– Reduce ProcArrayLock contention by removing backends in batches.
● PL/pgSQL
– Further reduce overhead for passing plpgsql variables to the executor.
● Planner / Optimizer
– Unique Joins
– Index-only scans with partial indexes
– FK join estimates
– Selectivity estimation for intarray
– Table Partition + Join Pushdown
– FDW join pushdown

Additional 9.6+ changes
● Declarative partitioning
– easier maintenance (huge improvement)
– allows advanced planning (insight into partitioning rules)
● Sorting
– Reusing abbreviated keys during second pass of ordered [set]
aggregates
– SortSupport for text - strcoll() and strxfrm() caching
– Memory prefetching while sequentially fetching from SortTuple
array, tuplestore
– Using quicksort and a merge step to significantly improve on
tuplesort's single run "external sort"

Performance improvements in PostgreSQL 9.5 and beyond

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