21 March 2019 0 comments PostgreSQL, Django
tl;dr; SELECT COUNT(*) FROM (SELECT DISTINCT my_not_unique_indexed_column FROM my_table) t;
I have a table that looks like this:
songsearch=# \d main_songtexthash
Table "public.main_songtexthash"
Column | Type | Collation | Nullable |
-----------+--------------------------+-----------+----------+
id | integer | | not null |
text_hash | character varying(32) | | not null |
created | timestamp with time zone | | not null |
modified | timestamp with time zone | | not null |
song_id | integer | | not null |
Indexes:
"main_songtexthash_pkey" PRIMARY KEY, btree (id)
"main_songtexthash_song_id_key" UNIQUE CONSTRAINT, btree (song_id)
"main_songtexthash_text_hash_c2771f1f" btree (text_hash)
"main_songtexthash_text_hash_c2771f1f_like" btree (text_hash varchar_pattern_ops)
Foreign-key constraints:
...snip...And the data looks something like this:
songsearch=# select text_hash, song_id from main_songtexthash limit 10;
text_hash | song_id
----------------------------------+---------
6f98e1945e64353bead9d6ab47a7f176 | 2565031
0c6662363aa4a340fea5efa24c98db76 | 486091
a25af539b183cbc338409c7acecc6828 | 212
5aaf561b38c251e7d863aae61fe1363f | 2141077
6a221df60f7cbb8a4e604f87c9e3aec0 | 245186
d2a0b5b3b33cdf5e03a75cfbf4963a6f | 1453382
95c395dd78679120269518b19187ca80 | 981402
8ab19b32b3be2d592aa69e4417b732cd | 616848
8ab19b32b3be2d592aa69e4417b732cd | 243393
01568f1f57aeb7a97e2544978fc93b4c | 333
(10 rows)If you look carefully, you'll notice that every song_id has a different text_hash except two of them.
Song IDs 616848 and 243393 both have the same text_hash of value 8ab19b32b3be2d592aa69e4417b732cd.
Also, if you imagine this table only has 10 rows, you could quickly and easily conclude that there are 10 different song_id but 9 different distinct text_hash. However, how do you do this counting if the tables are large??
The Wrong Way
songsearch=# select count(distinct text_hash) from main_songtexthash;
count
---------
1825983
(1 row)
And the explanation and cost analysis is:
songsearch=# explain analyze select count(distinct text_hash) from main_songtexthash;
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------
Aggregate (cost=44942.09..44942.10 rows=1 width=8) (actual time=40029.225..40029.226 rows=1 loops=1)
-> Seq Scan on main_songtexthash (cost=0.00..40233.87 rows=1883287 width=33) (actual time=0.029..193.653 rows=1879521 loops=1)
Planning Time: 0.059 ms
Execution Time: 40029.250 ms
(4 rows)Oh noes! A Sec Scan! Run!
The Right Way
Better explained in this blog post but basically, cutting to the chase, here's how you count on an indexed field:
songsearch=# select count(*) from (select distinct text_hash from main_songtexthash) t;
count
---------
1825983
(1 row)
And the explanation and cost analysis is:
songsearch=# explain analyze select count(*) from (select distinct text_hash from main_songtexthash) t;
QUERY PLAN
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Aggregate (cost=193871.20..193871.21 rows=1 width=8) (actual time=4894.555..4894.556 rows=1 loops=1)
-> Unique (cost=0.55..172861.54 rows=1680773 width=33) (actual time=0.075..4704.741 rows=1825983 loops=1)
-> Index Only Scan using main_songtexthash_text_hash_c2771f1f on main_songtexthash (cost=0.55..168153.32 rows=1883287 width=33) (actual time=0.074..4132.822 rows=1879521 loops=1)
Heap Fetches: 1879521
Planning Time: 0.082 ms
Execution Time: 4894.581 ms
(6 rows)Same exact result but ~5s instead of ~40s
. I'll take that, thank you very much.
The Django Way
As a bonus: Django is smart. Here's how they do it:
>>> SongTextHash.objects.values('text_hash').distinct().count()
1825983
And, the SQL it generates to make that count looks very familiar:
SELECT COUNT(*) FROM (SELECT DISTINCT "main_songtexthash"."text_hash" AS Col1 FROM "main_songtexthash") subquery
Conclusion
- Avoid "sequential scans" like the plague if you care about performance (...or not just killing your resources).
- Trust in Django.
22 February 2019
15 comments PostgreSQL, Python, Django, Web development
This an optimization story that should not surprise anyone using the Django ORM. But I thought I'd share because I have numbers now! The origin of this came from a real requirement. For a given parent model, I'd like to extract the value of the name column of all its child models, and the turn all these name strings into 1 MD5 checksum string.
Variants
The first attempted looked like this:
artist = Artist.objects.get(name="Bad Religion")
names = []
for song in Song.objects.filter(artist=artist):
names.append(song.name)
return hashlib.md5("".join(names).encode("utf-8")).hexdigest()
The SQL used to generate this is as follows:
SELECT "main_song"."id", "main_song"."artist_id", "main_song"."name",
"main_song"."text", "main_song"."language", "main_song"."key_phrases",
"main_song"."popularity", "main_song"."text_length", "main_song"."metadata",
"main_song"."created", "main_song"."modified",
"main_song"."has_lastfm_listeners", "main_song"."has_spotify_popularity"
FROM "main_song" WHERE "main_song"."artist_id" = 22729;
Clearly, I don't need anything but just the name column, version 2:
artist = Artist.objects.get(name="Bad Religion")
names = []
for song in Song.objects.filter(artist=artist).only("name"):
names.append(song.name)
return hashlib.md5("".join(names).encode("utf-8")).hexdigest()
Now, the SQL used is:
SELECT "main_song"."id", "main_song"."name"
FROM "main_song" WHERE "main_song"."artist_id" = 22729;
But still, since I don't really need instances of model class Song I can use the .values() method which gives back a list of dictionaries. This is version 3:
names = []
for song in Song.objects.filter(artist=a).values("name"):
names.append(song["name"])
return hashlib.md5("".join(names).encode("utf-8")).hexdigest()
This time Django figures it doesn't even need the primary key value so it looks like this:
SELECT "main_song"."name" FROM "main_song" WHERE "main_song"."artist_id" = 22729;
Last but not least; there is an even faster one. values_list(). This time it doesn't even bother to map the column name to the value in a dictionary. And since I only need 1 column's value, I can set flat=True. Version 4 looks like this:
names = []
for name in Song.objects.filter(artist=a).values_list("name", flat=True):
names.append(name)
return hashlib.md5("".join(names).encode("utf-8")).hexdigest()
Same SQL gets used this time as in version 3.
The benchmark
Hopefully this little benchmark script speaks for itself:
from songsearch.main.models import *
import hashlib
def f1(a):
names = []
for song in Song.objects.filter(artist=a):
names.append(song.name)
return hashlib.md5("".join(names).encode("utf-8")).hexdigest()
def f2(a):
names = []
for song in Song.objects.filter(artist=a).only("name"):
names.append(song.name)
return hashlib.md5("".join(names).encode("utf-8")).hexdigest()
def f3(a):
names = []
for song in Song.objects.filter(artist=a).values("name"):
names.append(song["name"])
return hashlib.md5("".join(names).encode("utf-8")).hexdigest()
def f4(a):
names = []
for name in Song.objects.filter(artist=a).values_list("name", flat=True):
names.append(name)
return hashlib.md5("".join(names).encode("utf-8")).hexdigest()
artist = Artist.objects.get(name="Bad Religion")
print(Song.objects.filter(artist=artist).count())
print(f1(artist) == f2(artist))
print(f2(artist) == f3(artist))
print(f3(artist) == f4(artist))
# Reporting
import time
import random
import statistics
functions = f1, f2, f3, f4
times = {f.__name__: [] for f in functions}
for i in range(500):
func = random.choice(functions)
t0 = time.time()
func(artist)
t1 = time.time()
times[func.__name__].append((t1 - t0) * 1000)
for name in sorted(times):
numbers = times[name]
print("FUNCTION:", name, "Used", len(numbers), "times")
print("\tBEST", min(numbers))
print("\tMEDIAN", statistics.median(numbers))
print("\tMEAN ", statistics.mean(numbers))
print("\tSTDEV ", statistics.stdev(numbers))
I ran this on my PostgreSQL 11.1 on my MacBook Pro with Django 2.1.7. So the database is on localhost.
The results
276
True
True
True
FUNCTION: f1 Used 135 times
BEST 6.309986114501953
MEDIAN 7.531881332397461
MEAN 7.834429211086697
STDEV 2.03779968066591
FUNCTION: f2 Used 135 times
BEST 3.039121627807617
MEDIAN 3.7298202514648438
MEAN 4.012803678159361
STDEV 1.8498943539073027
FUNCTION: f3 Used 110 times
BEST 0.9920597076416016
MEDIAN 1.4405250549316406
MEAN 1.5053835782137783
STDEV 0.3523240470133114
FUNCTION: f4 Used 120 times
BEST 0.9369850158691406
MEDIAN 1.3251304626464844
MEAN 1.4017681280771892
STDEV 0.3391019435930447
Discussion
I guess the hashlib.md5("".join(names).encode("utf-8")).hexdigest() stuff is a bit "off-topic" but I checked and it's roughly 300 times faster than building up the names list.
It's clearly better to ask less of Python and PostgreSQL to get a better total time. No surprise there. What was interesting was the proportion of these differences. Memorize that and you'll be better equipped if it's worth the hassle of not using the Django ORM in the most basic form.
Also, do take note that this is only relevant in when dealing with many records. The slowest variant (f1) takes, on average, 7 milliseconds.
Summarizing the difference with percentages compared to the fastest variant:
f1 - 573% slowerf2 - 225% slowerf3 - 6% slowerf4 - 0% slower
UPDATE Feb 25 2019
James suggested, although a bit "missing the point", that it could be even faster if all the aggregation is pushed into the PostgreSQL server and then the only thing that needs to transfer from PostgreSQL to Python is the final result.
By the way, name column in this particular benchmark, when concatenated into one big string, is ~4KB. So, with variant f5 it only needs to transfer 32 bytes which will/would make a bigger difference if the network latency is higher.
Here's the whole script: https://gist.github.com/peterbe/b2b7ed95d422ab25a65639cb8412e75e
And the results:
276
True
True
True
False
False
FUNCTION: f1 Used 92 times
BEST 5.928993225097656
MEDIAN 7.311463356018066
MEAN 7.594626882801885
STDEV 2.2027017044658423
FUNCTION: f2 Used 75 times
BEST 2.878904342651367
MEDIAN 3.3979415893554688
MEAN 3.4774907430013022
STDEV 0.5120246550765524
FUNCTION: f3 Used 88 times
BEST 0.9310245513916016
MEDIAN 1.1944770812988281
MEAN 1.3105544176968662
STDEV 0.35922655625999383
FUNCTION: f4 Used 71 times
BEST 0.7879734039306641
MEDIAN 1.1661052703857422
MEAN 1.2262606284987758
STDEV 0.3561764250427344
FUNCTION: f5 Used 90 times
BEST 0.7929801940917969
MEDIAN 1.0334253311157227
MEAN 1.1836051940917969
STDEV 0.4001442703048186
FUNCTION: f6 Used 84 times
BEST 0.80108642578125
MEDIAN 1.1119842529296875
MEAN 1.2281338373819988
STDEV 0.37146893005516973Result: f5 is takes 0.793ms and (the previous "winner") f4
takes 0.788ms.
I'm not entirely sure why f5 isn't faster but I suspect it's because the dataset is too small for it all to matter.
Compare:
songsearch=# explain analyze SELECT "main_song"."name" FROM "main_song" WHERE "main_song"."artist_id" = 22729;
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------
Index Scan using main_song_ca949605 on main_song (cost=0.43..229.33 rows=56 width=16) (actual time=0.014..0.208 rows=276 loops=1)
Index Cond: (artist_id = 22729)
Planning Time: 0.113 ms
Execution Time: 0.242 ms
(4 rows)with...
songsearch=# explain analyze SELECT md5(STRING_AGG("main_song"."name", '')) AS "names_hash" FROM "main_song" WHERE "main_song"."artist_id" = 22729;
QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------------
Aggregate (cost=229.47..229.48 rows=1 width=32) (actual time=0.278..0.278 rows=1 loops=1)
-> Index Scan using main_song_ca949605 on main_song (cost=0.43..229.33 rows=56 width=16) (actual time=0.019..0.204 rows=276 loops=1)
Index Cond: (artist_id = 22729)
Planning Time: 0.115 ms
Execution Time: 0.315 ms
(5 rows)I ran these two SQL statements about 100 times each and recorded their best possible execution times:
1) The plain SELECT - 0.99ms
2) The STRING_AGG - 1.06ms
So that accounts from ~0.1ms difference only! Which kinda matches the results seen above. All in all, I think the dataset is too small to demonstrate this technique. But, considering the chance that the complexity might not be linear with the performance benefit, it's still interesting.
Even though this tangent is a big off-topic, it is often a great idea to push as much work into the database as you can if applicable. Especially if it means you can transfer a lot less data eventually.
10 December 2018 2 comments PostgreSQL, MacOSX, Web development
It's weird to do performance analysis of a database you run on your laptop. When testing some app, your local instance probably has 1/1000 the amount of realistic data compared to a production server. Or, you're running a bunch of end-to-end integration tests whose PostgreSQL performance doesn't make sense to measure.
Anyway, if you are doing some performance testing of an app that uses PostgreSQL one great tool to use is pghero. I use it for my side-projects and it gives me such nice insights into slow queries that I'm willing to live with the cost that it is to run it on a production database.
This is more of a brain dump of how I run it locally:
First, you need to edit your postgresql.conf. Even if you used Homebrew to install it, it's not clear where the right config file is. Start psql
(on any database) and type this to find out which file is the one:
$ psql kintobench
kintobench=# show config_file;
config_file
-----------------------------------------
/usr/local/var/postgres/postgresql.conf
(1 row)
Now, open /usr/local/var/postgres/postgresql.conf and add the following lines:
# Peterbe: From Pghero's configuration help.
shared_preload_libraries = 'pg_stat_statements'
pg_stat_statements.track = all
Now, to restart the server use:
▶ brew services restart postgresql
Stopping `postgresql`... (might take a while)
==> Successfully stopped `postgresql` (label: homebrew.mxcl.postgresql)
==> Successfully started `postgresql` (label: homebrew.mxcl.postgresql)
The next thing you need is pghero itself and it's easy to run in docker. So to start, you need Docker for mac installed. You also need to know the database URL. Here's how I ran it:
docker run -ti -e DATABASE_URL=postgres://peterbe:@host.docker.internal:5432/kintobench -p 8080:8080 ankane/pghero
Note the trick of peterbe:@host.docker.internal because I don't use a password but inside the Docker container it doesn't know my terminal username. And the host.docker.internal is so the Docker container can reach the PostgreSQL installed on the host.
Once that starts up you can go to http://localhost:8080 in a browser and see a listing of all the cumulatively slowest queries. There are other cool features in pghero too that you can immediately benefit from such as hints about unused/redundent database indices.
Hope it helps!
19 April 2018 0 comments PostgreSQL, Python
tl;dr; Use best-explain-analyze.py to benchmark a SQL query in Postgres.
I often benchmark SQL by extracting the relevant SQL string, prefix it with EXPLAIN ANALYZE, putting it into a file (e.g. benchmark.sql) and then running psql mydatabase < benchmark.sql. That spits out something like this:
QUERY PLAN
--------------------------------------------------------------------------------------------------------------------------------
Index Scan using main_song_ca949605 on main_song (cost=0.43..237.62 rows=1 width=4) (actual time=1.586..1.586 rows=0 loops=1)
Index Cond: (artist_id = 27451)
Filter: (((name)::text % 'Facing The Abyss'::text) AND (id <> 2856345))
Rows Removed by Filter: 170
Planning time: 3.335 ms
Execution time: 1.701 ms
(6 rows)
Cool. So you study the steps of the query plan and look for "Seq Scan" and various sub-optimal uses of heaps and indices etc. But often, you really want to just look at the Execution time milliseconds number. Especially if you might have to slightly different SQL queries to compare and contrast.
However, as you might have noticed, the number on the Execution time
varies between runs. You might think nothing's changed but Postgres might have warmed up some internal caches or your host might be more busy or less busy. To remedy this, you run the EXPLAIN ANALYZE select ... a couple of times to get a feeling for an average. But there's a much better way!
best-explain-analyze.py
Check this out: best-explain-analyze.py
Download it into your ~/bin/ and chmod +x ~/bin/best-explain-analyze.py. I wrote it just this morning so don't judge!
Now, when you run it it runs that thing 10 times (by default) and reports the best Execution time, its mean and its median. Example output:
▶ best-explain-analyze.py songsearch dummy.sql
EXECUTION TIME
BEST 1.229ms
MEAN 1.489ms
MEDIAN 1.409ms
PLANNING TIME
BEST 1.994ms
MEAN 4.557ms
MEDIAN 2.292msThe "BEST" is an important metric. More important than mean or median.
Raymond Hettinger explains it better than I do. His context is for benchmarking Python code but it's equally applicable:
"Use the min() rather than the average of the timings. That is a recommendation from me, from Tim Peters, and from Guido van Rossum. The fastest time represents the best an algorithm can perform when the caches are loaded and the system isn't busy with other tasks. All the timings are noisy -- the fastest time is the least noisy. It is easy to show that the fastest timings are the most reproducible and therefore the most useful when timing two different implementations."
11 April 2018 1 comment PostgreSQL, Django, Python
The basic setup
Suppose you have these models:
from django.db import models
class Category(models.Model):
name = models.CharField(max_length=100)
class Blogpost(models.Model):
title = models.CharField(max_length=100)
categories = models.ManyToManyField(Category)
Suppose you hook these up Django REST Framework and list all Blogpost items. Something like this:
# urls.py
from rest_framework import routers
from . import views
router = routers.DefaultRouter()
router.register(r'blogposts', views.BlogpostViewSet)
# views.py
from rest_framework import viewsets
class BlogpostViewSet(viewsets.ModelViewSet):
queryset = Blogpost.objects.all().order_by('date')
serializer_class = serializers.BlogpostSerializer
What's the problem?
Then, if you execute this list (e.g. curl http://localhost:8000/api/blogposts/) what will happen, on the database, is something like this:
SELECT "app_blogpost"."id", "app_blogpost"."title" FROM "app_blogpost";
SELECT "app_category"."id", "app_category"."name" FROM "app_category" INNER JOIN "app_blogpost_categories" ON ("app_category"."id" = "app_blogpost_categories"."category_id") WHERE "app_blogpost_categories"."blogpost_id" = 1025;
SELECT "app_category"."id", "app_category"."name" FROM "app_category" INNER JOIN "app_blogpost_categories" ON ("app_category"."id" = "app_blogpost_categories"."category_id") WHERE "app_blogpost_categories"."blogpost_id" = 193;
SELECT "app_category"."id", "app_category"."name" FROM "app_category" INNER JOIN "app_blogpost_categories" ON ("app_category"."id" = "app_blogpost_categories"."category_id") WHERE "app_blogpost_categories"."blogpost_id" = 757;
SELECT "app_category"."id", "app_category"."name" FROM "app_category" INNER JOIN "app_blogpost_categories" ON ("app_category"."id" = "app_blogpost_categories"."category_id") WHERE "app_blogpost_categories"."blogpost_id" = 853;
SELECT "app_category"."id", "app_category"."name" FROM "app_category" INNER JOIN "app_blogpost_categories" ON ("app_category"."id" = "app_blogpost_categories"."category_id") WHERE "app_blogpost_categories"."blogpost_id" = 1116;
SELECT "app_category"."id", "app_category"."name" FROM "app_category" INNER JOIN "app_blogpost_categories" ON ("app_category"."id" = "app_blogpost_categories"."category_id") WHERE "app_blogpost_categories"."blogpost_id" = 1126;
SELECT "app_category"."id", "app_category"."name" FROM "app_category" INNER JOIN "app_blogpost_categories" ON ("app_category"."id" = "app_blogpost_categories"."category_id") WHERE "app_blogpost_categories"."blogpost_id" = 964;
SELECT "app_category"."id", "app_category"."name" FROM "app_category" INNER JOIN "app_blogpost_categories" ON ("app_category"."id" = "app_blogpost_categories"."category_id") WHERE "app_blogpost_categories"."blogpost_id" = 591;
SELECT "app_category"."id", "app_category"."name" FROM "app_category" INNER JOIN "app_blogpost_categories" ON ("app_category"."id" = "app_blogpost_categories"."category_id") WHERE "app_blogpost_categories"."blogpost_id" = 1112;
SELECT "app_category"."id", "app_category"."name" FROM "app_category" INNER JOIN "app_blogpost_categories" ON ("app_category"."id" = "app_blogpost_categories"."category_id") WHERE "app_blogpost_categories"."blogpost_id" = 1034;
...
Obviously, it depends on how you define that serializers.BlogpostSerializer class, but basically, as it loops over the Blogpost, for each and every one, it needs to make a query to the many-to-many table (app_blogpost_categories in this example).
That's not going to be performant. In fact, it might be dangerous on your database if the query of blogposts gets big, like requesting a 100 or 1,000 records. Fetching 1,000 rows from the app_blogpost table might be cheap'ish but doing 1,000 selects with JOIN is never going to be cheap. It adds up horribly.
How you solve it
The trick is to only do 1 query on the many-to-many field's table, 1 query on the app_blogpost table and 1 query on the app_category table.
First you have to override the ViewSet.list method. Then, in there you can do exactly what you need.
Here's the framework for this change:
# views.py
from rest_framework import viewsets
class BlogpostViewSet(viewsets.ModelViewSet):
# queryset = Blogpost.objects.all().order_by('date')
serializer_class = serializers.BlogpostSerializer
def get_queryset(self):
# Chances are, you're doing something more advanced here
# like filtering.
Blogpost.objects.all().order_by('date')
def list(self, request, *args, **kwargs):
response = super().list(request, *args, **kwargs)
# Where the magic happens!
return response
Next, we need to make a mapping of all Category.id -1-> Category.name. But we want to make sure we do only on the categories that are involved in the Blogpost records that matter. You could do something like this:
category_names = {}
for category in Category.objects.all():
category_names[category.id] = category.name
But to avoid doing a lookup of category names for those you never need, use the query set on Blogpost. I.e.
qs = self.get_queryset()
all_categories = Category.objects.filter(
id__in=Blogpost.categories.through.objects.filter(
blogpost__in=qs
).values('category_id')
)
category_names = {}
for category in all_categories:
category_names[category.id] = category.name
Now you have a dictionary of all the Category IDs that matter.
Note! The above "optimization" assumes that it's worth it. Meaning, if the number of Category records in your database is huge, and the Blogpost queryset is very filtered, then it's worth only extracting a subset. Alternatively, if you only have like 100 different categories in your database, just do the first variant were you look them up "simplestly" without any fancy joins.
Next, is the mapping of Blogpost.id -N-> Category.name. To do that you need to build up a dictionary (int to list of strings). Like this:
categories_map = defaultdict(list)
for m2m in Blogpost.categories.through.objects.filter(blogpost__in=qs):
categories_map[m2m.blogpost_id].append(
category_names[m2m.category_id]
)
So what we have now is a dictionary whose keys are the IDs in self.get_queryset() and each value is a list of a strings. E.g. ['Category X', 'Category Z'] etc.
Lastly, we need to put these back into the serialized response. This feels a little hackish but it works:
for each in response.data:
each['categories'] = categories_map.get(each['id'], [])
The whole solution looks something like this:
# views.py
from rest_framework import viewsets
class BlogpostViewSet(viewsets.ModelViewSet):
# queryset = Blogpost.objects.all().order_by('date')
serializer_class = serializers.BlogpostSerializer
def get_queryset(self):
# Chances are, you're doing something more advanced here
# like filtering.
Blogpost.objects.all().order_by('date')
def list(self, request, *args, **kwargs):
response = super().list(request, *args, **kwargs)
qs = self.get_queryset()
all_categories = Category.objects.filter(
id__in=Blogpost.categories.through.objects.filter(
blogpost__in=qs
).values('category_id')
)
category_names = {}
for category in all_categories:
category_names[category.id] = category.name
categories_map = defaultdict(list)
for m2m in Blogpost.categories.through.objects.filter(blogpost__in=qs):
categories_map[m2m.blogpost_id].append(
category_names[m2m.category_id]
)
for each in response.data:
each['categories'] = categories_map.get(each['id'], [])
return response
It's arguably not very pretty but doing 3 tight queries instead of doing as many queries as you have records is much better. O(c) is better than O(n).
Discussion
Perhaps the best solution is to not run into this problem. Like, don't serialize any many-to-many fields.
Or, if you use pagination very conservatively, and only allow like 10 items per page then it won't be so expensive to do one query per every many-to-many field.
07 February 2018 0 comments PostgreSQL
tl;dr;
SELECT pg_database.datname, pg_database_size(pg_database.datname), pg_size_pretty(pg_database_size(pg_database.datname)) FROM pg_database ORDER by 2 DESC;
I recently had to transfer all my postgres data for my local databases on the laptop. Although I, unfortunately, can't remember what it was before I deleted a bunch of databases, now after some clean up, all my PostgreSQL databases weighs 12GB.
To find out the size of all your database, start psql like this:
$ psql postgres
Then run this:
SELECT pg_database.datname,
pg_database_size(pg_database.datname),
pg_size_pretty(pg_database_size(pg_database.datname))
FROM pg_database ORDER by 2 DESC;
Here's what it looked like on my laptop:
postgres=# SELECT pg_database.datname,
postgres-# pg_database_size(pg_database.datname),
postgres-# pg_size_pretty(pg_database_size(pg_database.datname))
postgres-# FROM pg_database ORDER by 2 DESC;
datname | pg_database_size | pg_size_pretty
--------------------------+------------------+----------------
songsearch | 10689224876 | 10194 MB
air_mozilla_org | 355639812 | 339 MB
kl | 239297028 | 228 MB
kl2 | 239256068 | 228 MB
peterbecom | 191914500 | 183 MB
kintobench | 125968556 | 120 MB
airmozilla | 41640452 | 40 MB
socorro_webapp | 32530948 | 31 MB
tecken | 26706092 | 25 MB
dailycookie | 12935684 | 12 MB
autocompeter | 12313092 | 12 MB
socorro_integration_test | 11428356 | 11 MB
breakpad | 11313668 | 11 MB
test_peterbecom | 9298436 | 9081 kB
battleshits | 9028100 | 8817 kB
thuawood2 | 8716804 | 8513 kB
thuawood | 8667652 | 8465 kB
fastestdb | 8012292 | 7825 kB
premailer | 7676420 | 7497 kB
crontabber | 7586308 | 7409 kB
postgres | 7536812 | 7360 kB
crontabber_exampleapp | 7488004 | 7313 kB
whatsdeployed | 7414276 | 7241 kB
socorro_test | 7315972 | 7145 kB
template1 | 7307780 | 7137 kB
template0 | 7143940 | 6977 kB
(26 rows)
