Parquet and Postgres in the Data Lake
Static Data is Different
A couple weeks ago, I came across a blog from Retool on their experience migrating a 4TB database. They put in place some good procedures and managed a successful migration, but the whole experience was complicated by the size of the database. The size of the database was the result of a couple of very large "logging" tables: an edit log and an audit log.
The thing about log tables is, they don't change much. They are append-only by design. They are also queried fairly irregularly, and the queries are often time ranged: "tell me what happened then" or "show me the activities between these dates".
So, one way the Retool migration could have been easier is if their log tables were constructed as time-ranged partitions. That way there'd would only be one "live" table in the partition set (the one with the recent entries) and a larger collection of historical tables. The migration could move the live partition as part of the critical path, and do all the historical partitions later.
Even after breaking up the log tables into manageable chunks they still remain, in aggregate, pretty big! The PostgreSQL documentation on partitioning has some harsh opinions about stale data living at the end of a partition collection:
The simplest option for removing old data is to drop the partition that is no longer necessary.
There's something to that! All those old historical records just fluff up your base backups, and maybe you almost never have occasion to query it.
Is there an alternative to dropping the tables?
Dump Your Data in the Lake
What if there was a storage option that was still durable, allowed access via multiple query tools, and could integrate transparently into your operational transactional database?
Sounds a bit crazy, but:
- A foreign parquet table can participate in a partition along with a native PostgreSQL table.
- A parquet file can also be consumed by R, Python, Go and a host of cloud applications.
- Modern PostgreSQL (14+) can parallelize access to foreign tables, so even collections of Parquet files can be scanned effectively.
- Parquet stores data compressed, so you can get way more raw data into less storage.
Parquet is a language-independent storage format, designed for online analytics, so:
- Column oriented
A standard table in PostgreSQL will be row-oriented on disk.
This layout is good for things PostgreSQL is expected to do, like query, insert, update and delete data a "few" records at a time. (The value of "a few" can run into the hundreds of thousands or millions, depending on the operation.)
A Parquet file stores data column-oriented on the disk, in batches called "row groups".
You can see where the Parquet format gets its name: the data is grouped into little squares, like a parquet floor. One of the advantages of grouping data together, is that compression routines tend to work better on data of the same type, and even more so when the data elements have the same values.
Does This Even Work?
In a word "yes", but with some caveats: Parquet has been around for several years, but the ecosystem supporting it is still, relatively, in flux. New releases of the underlying C++ libraries are still coming out regularly, the parquet_fdw is only a couple years old, and so on.
However, I was able to demonstrate to my own satisfaction that things were baked enough to be interesting.
I started with a handy data table of Philadelphia parking infractions, that I
used in a previous
blog post on spatial joins,
and sorted the file by date of infraction,
Data Download and Sort
# # Download Philadelphia parking infraction data # curl "https://phl.carto.com/api/v2/sql?filename=parking_violations&format=csv&skipfields=cartodb_id,the_geom,the_geom_webmercator&q=SELECT%20*%20FROM%20parking_violations%20WHERE%20issue_datetime%20%3E=%20%272012-01-01%27%20AND%20issue_datetime%20%3C%20%272017-12-31%27" > phl_parking_raw.csv # # Sort it # sort -k2 -t, phl_parking_raw.csv > phl_parking.csv
Sorting the data by
issue_datetime will make queries that filter against that
column go faster in the column-oriented Parquet setup.
-- Create parking infractions table CREATE TABLE phl_parking ( anon_ticket_number integer, issue_datetime timestamptz, state text, anon_plate_id integer, division text, location text, violation_desc text, fine float8, issuing_agency text, lat float8, lon float8, gps boolean, zip_code text ); -- Read in the parking data \copy phl_parking FROM 'phl_parking.csv' WITH (FORMAT csv, HEADER true);
OK, so now I have an 8M record data table, good for some bulk data experiments. How big is the table?
SELECT pg_size_pretty(pg_relation_size('phl_parking')) AS pg_table_size;
pg_table_size ---------------- 1099 MB
Just over 1GB!
How do I get a Parquet file?
This turns out to be way harder than I expected. Most internet advice was around
using Python or Spark to convert CSV files into Parquet. In the end, I used the
very new (currently unreleased, coming in GDAL 3.5)
support for Parquet in GDAL library,
ogr2ogr command to do the conversion.
ogr2ogr -f Parquet \ /tmp/phl_parking.parquet \ PG:"dbname=phl host=localhost" \ phl_parking
For these tests the Parquet file will reside on my local disk in
for cloud purposes it might reside on a cloud volume, or even (with the
right software) in an object
% ls -lh /tmp/phl_parking.parquet -rw-r--r-- 1 pramsey wheel 216M 29 Apr 10:44 /tmp/phl_parking.parquet
Thanks to data compression, the Parquet file is 20% the size of the database table!
Querying Parquet in PostgreSQL involves a number of parts, which can be challenging to build right now.
libarrow version 6, not the recently released
Once the FDW and supporting libraries are built, though, everything works just like other FDW extensions.
CREATE EXTENSION parquet_fdw; CREATE SERVER parquet_srv FOREIGN DATA WRAPPER parquet_fdw; CREATE FOREIGN TABLE phl_parking_pq ( anon_ticket_number integer, issue_datetime timestamptz, state text, anon_plate_id integer, division text, location text, violation_desc text, fine float8, issuing_agency text, lat float8, lon float8, gps boolean, zip_code text ) SERVER parquet_srv OPTIONS (filename '/tmp/phl_parking.parquet', sorted 'issue_datetime', use_threads 'true');
Compared to the raw table, the Parquet file is similar in performance, usually a little slower. Just blasting through a row count (when the tables are pre-cached in memory).
-- Give native table same indexing advantage -- as the parquet file CREATE INDEX ON phl_parking USING BRIN (issue_datetime); SELECT Count(*) FROM phl_parking_pq; -- Time: 1230 ms SELECT Count(*) FROM phl_parking; -- Time: 928 ms
Similarly, a filter also is slightly faster on PostgreSQL.
SELECT Sum(fine), Count(1) FROM phl_parking_pq WHERE issue_datetime BETWEEN '2014-01-01' AND '2015-01-01'; -- Time: 692 ms SELECT Sum(fine), Count(1) FROM phl_parking WHERE issue_datetime BETWEEN '2014-01-01' AND '2015-01-01'; -- Time: 571 ms
parquet_fdw is very nicely implemented, and will even tell you the
execution plan that will be used on the file for a given filter. For example,
the previous filter involves opening about 20% of the 132 row groups in the
EXPLAIN SELECT Sum(fine), Count(1) FROM phl_parking_pq WHERE issue_datetime BETWEEN '2014-01-01' AND '2015-01-01'; Finalize Aggregate (cost=6314.77..6314.78 rows=1 width=16) -> Gather (cost=6314.55..6314.76 rows=2 width=16) Workers Planned: 2 -> Partial Aggregate (cost=5314.55..5314.56 rows=1 width=16) -> Parallel Foreign Scan on phl_parking_pq (cost=0.00..5242.88 rows=14333 width=8) Filter: ((issue_datetime >= '2014-01-01 00:00:00-08') AND (issue_datetime <= '2015-01-01 00:00:00-08')) Reader: Single File Row groups: 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63
For plowing through the whole table and doing a summary, the Parquet query is about the same speed as the PostgreSQL query.
SELECT issuing_agency, count(1) FROM phl_parking_pq GROUP BY issuing_agency; -- Time: 3043 ms SELECT issuing_agency, count(1) FROM phl_parking GROUP BY issuing_agency; -- Time: 3103 ms
My internal model for the performance differences is that, while the Parquet format has some advantages in avoiding unnecessary reads, via row block filtering and accessing only the columns of interest, those advantages are offset by some inefficiencies in converting the raw data from parquet into the internal PostgreSQL formats.
Is that it? Well, we've seen:
- Parquet is a software-neutral format that is increasingly common in data science and the data centre.
- Parquet access can be made transparent to PostgreSQL via the
- Parquet storage can provide substantial space savings.
- Parquet storage is a bit slower than native storage, but can offload management of static data from the back-up and reliability operations needed by the rest of the data.
- Parquet storage of static data is much better than just throwing it out.
More importantly, I think there's more to discuss:
- Can parquet files participate in partitions?
- Can parquet files be accessed in parallel in collections?
- Can parquet files reside in cloud object storage instead of filesystem storage?
- Can PostgreSQL with parquet storage act like a "mid-range big data engine" to crunch numbers on large collections of static data backed by parquet?
So far the ecosystem of Parquet tools has been dominated by the needs of data science (R and Python) and a handful of cloud OLAP system (Apache Spark), but there's no reason PostgreSQL can't start to partake of this common cloud format goodness, and start to swim in the data lake.
May 3, 2022 •More by this author