Introduction

Redshift is a column-store relational database, which means that each column in a table is stored independently.

It is often the case that the data in a single column has similar characteristics - for example, it might be all names, or ages, or might be integer values within a given range; in other words, data which is far from randomly distributed across the value range for the data type of the column.

This provides an opportunity for unusually effective data compression, as well as an opportunity to blunder terribly, as Redshift offers a range of data compression methods (known in the Redshift documentation as “encodings”), most of which work spectacularly well with and only with data which expresses the suitable characteristics for that data compression method (and spectacularly badly with data which lacks those suitable characteristics).

It is then necessary to understand the type of data characteristics suitable for each of the data compression methods offered by Redshift, as well of course as the properties, behaviours and limitations of the data compression methods, so the good choices can be made when selecting data compression for columns.

This document is one in a series, each of which examines one data compression method offered by Redshift, which here investigates the raw encoding.

(I normally do not work, but currently I am on a full-time contract, so if I spent the time necessary to produce a single document with all encodings, you would hear nothing from me for many weeks. Once all the encodings have been documented, a single document, “Encodings”, will be released.)

Test Method

First, the encodings available to Redshift are enumerated by using the function format_encoding( int4 ). This function takes an argument between 0 and 255 (despite taking an int4) which is the ID of an encoding, and returns a string which is the name of the encoding, or the string “unknown”, if the ID has no matching encoding.

Second, we then enumerate which data types can use which encodings, by obtaining a complete list of data types from pg_type, and making a table for every combination of data type and encoding, and noting which combinations are permitted and which are not.

(For char and varchar, we use a small selection of variants, where we independently vary both the DDL lengths and actual length of the strings. Note that geometry, hllsketch and super are not investigated, because I’ve not yet learned about them, so I have only superficial knowledge of how they work, not enough to investigate them here. When I do, I’ll update this document.)

Third, we then find how many rows of each raw encoded data type fit into a single block.

Results

The results are given here for ease of reference, but they are primarily presented, piece by piece along with explanation, in the Discussion.

See Appendix A for the Python pprint dump of the results dictionary.

The script used to generated these results in designed to readers to use, and is available here.

Test duration, excluding server bring-up and shut-down, was 1641 seconds.

dc2.large, 2 nodes (1.0.32574)

Enumerated Encodings

Valid Encodings for Data Types

Valid Data Types for Encodings

raw Encoding

Discussion

To begin with, we enumerate all the encodings Redshift knows about.

There’s a function, format_encoding( int4 ), which takes a single int4 argument which is an encoding ID (which range from 0 to 255 - outside this range and you get an error), and returns a string which is the name of the encoding, or “unknown” if there is no encoding for the given ID.

Setting aside all “unknown”s, we find the following;

There are a couple of items of note;

1. none means raw.
2. Some encodings have more than one ID.
3. There’s a set of three globaldict encodings which are not mentioned in the documentation.

Next, let’s check to see which data types can use which encodings.

(We can only specify encodings by their names, so we can’t try to use the different IDs of encodings with multiple IDs. Also note normally I always use Redshift internal names, so say int8, which is the name you find in the system tables, rather than bigint, which is an alias, but with raw encoding the internal name is none but you can’t use that with CREATE TABLE - it only understands raw.)

Valid Encodings for Data Types

Valid Data Types for Encodings

The docs page for which encodings support which data types is here. It seems evidently hand-maintained, as it is out of date.

1. All encodings which can support time and timetz (az64, bytedict, delta, delta32k, lzo, runlength, zstd) are missing support for those two data types.
2. delta and delta32k support for timestamptz is missing.
3. lzo and zstd support for super is missing.

I may be wrong, but it seems obvious to me any serious documentation for a continually evolving software product must at least in part to be automatically generated if it is to avoid becoming increasingly inaccurate over time.

It also would seem if there’s any ongoing checking of the docs, it is ineffective, since we see here the most simple, basic and fundamental information is inaccurate.

Having then set the scene, both as you will see for the quality of the documentation, as well as for encodings, let us turn to each encoding in turn, and see for each what we can find out.

We turn now to the raw encoding.

Being what it is, there’s nothing to say about how raw encodes - but by being raw, by doing nothing, it allows us to examine other properties of storing rows in blocks. In particular, a critical question turns out to be how many rows of each data type fit into a single block.

This turns out to be an excellent question, because we find that first, NULL or NOT NULL matters, and, secondly, it’s not just a case of there being as many rows as will fit in one megabyte; there’s some unused space, and it seems to be the longer the data type, the more unused space there is.

Here we see the number of values stored per block when NOT NULL is set “(NN)”, the number when NULL is set “(N)” and the difference between the two.

The first and most startling observation is the huge number of values stored by boolean, which must be 1 bit per value to be storing 8.3m values in a one megabyte block. We also note that setting NULL (as opposed to NOT NULL) roughly halves the number of values - clearly, a 1 bit flag per value is used to indicate whether a value is NULL or not. More on this below.

The official documentation for boolean, found here, states boolean is 1 byte per value, and, what’s more, that it is 1 byte whether true, false or NULL, which is not just wrong, but also misleads readers as to how NULL is handled.

Moving on to char, we can see that the maximum length of the char in the DDL determines the store required; the actual length of the string is not relevant. When we get to char(4096), there are only 248 values in a block; each when NULL requires one more bit to store, but 248 bits is small enough that it makes no difference to the number of values which can be stored.

The numeric type is worth a mention, in that precision 1 to 19 gives an 8 byte value, precision 20 to 38 gives a 16 byte value. This is why I select the precisions 1, 19, 20 and 38, to demonstrate the transition.

Finally, coming to varchar, we find that this data type requires 1 byte, rather than 1 bit, to indicate NULL.

So, now we now directly from STV_BLOCKLIST how many values are in a block, both for NOT NULL and NULL. We can then divide the size of the block by the number of values, to see how many bits are being used per value.

There is in fact always some unused space, but the number of bits must be an integer, so if we end up with say 32.2 bits being used per value, then the number of bits must be 32, and we can compute the amount of unused space from the fractional part of the number.

Note here we have bits per value, but bytes of unused space.

So, quite a few matters to note;

1. boolean is 1 bit per value
2. char always uses the maximum length specified in the DDL
3. all data types, except varchar, use one additional bit per value if the column is NULL (as opposed to NOT NULL), which will matter for small data types once you get into Big Data
4. varchar stores only the actual length of the string, plus a four byte header (which presumably indicates length)
5. varchar uses 1 byte per value if the column is NULL
6. As mentioned in a previous white paper, numeric is 8 bytes up to precision 19, then becomes 16 bytes. The actual value stored makes no difference; it is and only is the DDL which determines the data type length.

So it is then that a varchar(1) NULL is 48 bits in length, carrying 8 bits of data. Don’t do that - if you don’t need UTF-8, use a char(1) NULL, which is 9 bits per value.

Now, we computed the unused space by dividing the size of the block by the number of values, to see how many bits are being used per value. However, if the number of values stored in one block is the same for both NULL and NOT NULL (as happens with the long char and varchar data types), this approach partially fails, in that it ends up thinking the amount of unused space is the same in both cases - we already known, from what we’ve seen above, that this is not so. All that’s actually happening is the overheads of handling NULL are so small, given the very small number of values, that they do not change the number of values which can be stored in one block.

In these special cases, the four of them below, I have in the script manually specified the number of bits per value, based on the knowledge from the table above, and then computed the unused space for NULL and NOT NULL.

What’s interesting here is that the amount of unused space is large. For varchar it makes sense - the space remaining is non-trivial, but it’s always smaller than the amount needed for one more value to be stored - but for char, it doesn’t make sense. A char(4096) has 32,768 unused bytes in each block. There are 248 values being stored, another 8 values could be stored (7 if NULL). What gives?

Well, I have a bit of a suspicion this - the unused space - is being done to improve VACUUM performance. If a user inserts only a few rows, you can maybe get away with only needing to resort the individual blocks which each take some of the new rows, because they have room to take them; it saves you needing to resort every block after the blocks which take new rows, which you would have to do if each block was already completely full.

Conclusions

The official documentation is out of date with regard to which encodings support which data types.

1. All encodings which can support time and timetz (az64, bytedict, delta, delta32k, lzo, runlength, zstd) are missing support for those two data types.
2. delta and delta32k support for timestamptz is missing.
3. lzo and zstd support for super is missing.

The boolean data type is 1 bit in size (the documentation states 1 byte; this is incorrect).

Setting a column to NULL (as opposed to NOT NULL) requires an additional 1 bit of store per value, except for varchar, which requires an additional 1 byte of store per value.

Blocks when full, in the sense that an additional value will lead to a new block being formed, have a little unused space. The amount varies by data type, and increases as the data type becomes larger (in terms of bytes per value). Typically the unused space is small, on the order of hundreds of bytes, but for long char and varchar strings (remembering that char always uses the full length of the DDL length, but varchar only uses the actual length of the string, plus a four byte length header) the unused space becomes larger, with char(4096) leaving 32737 bytes unused and varchar(65535) with a full length string leaving 65,476 bytes unused (the latter being understandable, as there is not enough room for another value).

I have a suspicion the unused space is to help with VACUUM performance in certain situations, but it’s a guess.

Revision History

v1

• Initial release.

v2

• Added “About the Author”. made site name in title a link, and made each chapter start a new page.
• Updated links to amazonredshiftresearcproject.org to redshiftresearcproject.org.

v3

• Web-site name changed to “Redshift Observatory”.
• Updated links from redshiftresearcproject.org to redshift-observatory.ch.

v4

• Cosmetic fixes - accidental use of single quotes rather than backticks.

v5

• Removed “About The Author”.
• Added Slack join URL.

Appendix A : Raw Data Dump

Note these results are completely unprocessed; they are a raw dump of the results, so the original, wholly unprocessed data, is available.

{'proofs': {'dc2.large': {2: {'data_type_encodings': {'bool': ['raw',
                                                               'runlength',
                                                               'zstd'],
                                                      'bpchar': ['bytedict',
                                                                 'lzo',
                                                                 'raw',
                                                                 'runlength',
                                                                 'zstd'],
                                                      'char': ['bytedict',
                                                               'lzo',
                                                               'raw',
                                                               'runlength',
                                                               'zstd'],
                                                      'date': ['az64',
                                                               'bytedict',
                                                               'delta',
                                                               'delta32k',
                                                               'lzo',
                                                               'raw',
                                                               'runlength',
                                                               'zstd'],
                                                      'float4': ['bytedict',
                                                                 'raw',
                                                                 'runlength',
                                                                 'zstd'],
                                                      'float8': ['bytedict',
                                                                 'raw',
                                                                 'runlength',
                                                                 'zstd'],
                                                      'geometry': ['raw'],
                                                      'hllsketch': ['raw'],
                                                      'int2': ['az64',
                                                               'bytedict',
                                                               'delta',
                                                               'lzo',
                                                               'mostly8',
                                                               'raw',
                                                               'runlength',
                                                               'zstd'],
                                                      'int4': ['az64',
                                                               'bytedict',
                                                               'delta',
                                                               'delta32k',
                                                               'lzo',
                                                               'mostly16',
                                                               'mostly8',
                                                               'raw',
                                                               'runlength',
                                                               'zstd'],
                                                      'int8': ['az64',
                                                               'bytedict',
                                                               'delta',
                                                               'delta32k',
                                                               'lzo',
                                                               'mostly16',
                                                               'mostly32',
                                                               'mostly8',
                                                               'raw',
                                                               'runlength',
                                                               'zstd'],
                                                      'numeric': ['az64',
                                                                  'bytedict',
                                                                  'delta',
                                                                  'delta32k',
                                                                  'lzo',
                                                                  'mostly16',
                                                                  'mostly32',
                                                                  'mostly8',
                                                                  'raw',
                                                                  'runlength',
                                                                  'zstd'],
                                                      'super': ['lzo',
                                                                'raw',
                                                                'zstd'],
                                                      'text': ['bytedict',
                                                               'lzo',
                                                               'raw',
                                                               'runlength',
                                                               'text32k',
                                                               'zstd'],
                                                      'time': ['az64',
                                                               'bytedict',
                                                               'delta',
                                                               'delta32k',
                                                               'lzo',
                                                               'raw',
                                                               'runlength',
                                                               'zstd'],
                                                      'timestamp': ['az64',
                                                                    'bytedict',
                                                                    'delta',
                                                                    'delta32k',
                                                                    'lzo',
                                                                    'raw',
                                                                    'runlength',
                                                                    'zstd'],
                                                      'timestamptz': ['az64',
                                                                      'bytedict',
                                                                      'delta',
                                                                      'delta32k',
                                                                      'lzo',
                                                                      'raw',
                                                                      'runlength',
                                                                      'zstd'],
                                                      'timetz': ['az64',
                                                                 'bytedict',
                                                                 'delta',
                                                                 'delta32k',
                                                                 'lzo',
                                                                 'raw',
                                                                 'runlength',
                                                                 'zstd'],
                                                      'varchar': ['bytedict',
                                                                  'lzo',
                                                                  'raw',
                                                                  'runlength',
                                                                  'text255',
                                                                  'text32k',
                                                                  'zstd']},
                              'data_type_values_per_block': [('boolean',
                                                              8387697,
                                                              4193849,
                                                              4193848,
                                                              ''),
                                                             ('char(0001)',
                                                              1048463,
                                                              931967,
                                                              116496,
                                                              'one character '
                                                              'string'),
                                                             ('char(0008)',
                                                              131051,
                                                              129035,
                                                              2016,
                                                              'one character '
                                                              'string'),
                                                             ('char(0008)',
                                                              131051,
                                                              129035,
                                                              2016,
                                                              'full length '
                                                              'string'),
                                                             ('char(0064)',
                                                              16375,
                                                              16343,
                                                              32,
                                                              'one character '
                                                              'string'),
                                                             ('char(0064)',
                                                              16375,
                                                              16343,
                                                              32,
                                                              'full length '
                                                              'string'),
                                                             ('char(4096)',
                                                              248,
                                                              248,
                                                              0,
                                                              'one character '
                                                              'string'),
                                                             ('char(4096)',
                                                              248,
                                                              248,
                                                              0,
                                                              'full length '
                                                              'string'),
                                                             ('date',
                                                              262085,
                                                              254143,
                                                              7942,
                                                              ''),
                                                             ('float4',
                                                              262085,
                                                              254143,
                                                              7942,
                                                              ''),
                                                             ('float8',
                                                              130994,
                                                              128978,
                                                              2016,
                                                              ''),
                                                             ('int2',
                                                              524219,
                                                              493382,
                                                              30837,
                                                              ''),
                                                             ('int4',
                                                              262085,
                                                              254143,
                                                              7942,
                                                              ''),
                                                             ('int8',
                                                              130994,
                                                              128978,
                                                              2016,
                                                              ''),
                                                             ('time',
                                                              130994,
                                                              128978,
                                                              2016,
                                                              ''),
                                                             ('timestamp',
                                                              130994,
                                                              128978,
                                                              2016,
                                                              ''),
                                                             ('timestamptz',
                                                              130994,
                                                              128978,
                                                              2016,
                                                              ''),
                                                             ('timetz',
                                                              130994,
                                                              128978,
                                                              2016,
                                                              ''),
                                                             ('numeric(1,0)',
                                                              130994,
                                                              128978,
                                                              2016,
                                                              ''),
                                                             ('numeric(19,0)',
                                                              130994,
                                                              128978,
                                                              2016,
                                                              ''),
                                                             ('numeric(20,0)',
                                                              65401,
                                                              64894,
                                                              507,
                                                              ''),
                                                             ('numeric(38,0)',
                                                              65401,
                                                              64894,
                                                              507,
                                                              ''),
                                                             ('varchar(00001)',
                                                              209694,
                                                              174745,
                                                              34949,
                                                              'one character '
                                                              'string'),
                                                             ('varchar(00008)',
                                                              209694,
                                                              174745,
                                                              34949,
                                                              'one character '
                                                              'string'),
                                                             ('varchar(00008)',
                                                              87372,
                                                              80651,
                                                              6721,
                                                              'full length '
                                                              'string'),
                                                             ('varchar(00064)',
                                                              209694,
                                                              174745,
                                                              34949,
                                                              'one character '
                                                              'string'),
                                                             ('varchar(00064)',
                                                              15418,
                                                              15195,
                                                              223,
                                                              'full length '
                                                              'string'),
                                                             ('varchar(04096)',
                                                              209694,
                                                              174745,
                                                              34949,
                                                              'one character '
                                                              'string'),
                                                             ('varchar(04096)',
                                                              255,
                                                              255,
                                                              0,
                                                              'full length '
                                                              'string'),
                                                             ('varchar(65535)',
                                                              209694,
                                                              174745,
                                                              34949,
                                                              'one character '
                                                              'string'),
                                                             ('varchar(65535)',
                                                              15,
                                                              15,
                                                              0,
                                                              'full length '
                                                              'string')],
                              'encodings_data_type': {'az64': ['date',
                                                               'int2',
                                                               'int4',
                                                               'int8',
                                                               'numeric',
                                                               'time',
                                                               'timestamp',
                                                               'timestamptz',
                                                               'timetz'],
                                                      'bytedict': ['bpchar',
                                                                   'char',
                                                                   'date',
                                                                   'float4',
                                                                   'float8',
                                                                   'int2',
                                                                   'int4',
                                                                   'int8',
                                                                   'numeric',
                                                                   'text',
                                                                   'time',
                                                                   'timestamp',
                                                                   'timestamptz',
                                                                   'timetz',
                                                                   'varchar'],
                                                      'delta': ['date',
                                                                'int2',
                                                                'int4',
                                                                'int8',
                                                                'numeric',
                                                                'time',
                                                                'timestamp',
                                                                'timestamptz',
                                                                'timetz'],
                                                      'delta32k': ['date',
                                                                   'int4',
                                                                   'int8',
                                                                   'numeric',
                                                                   'time',
                                                                   'timestamp',
                                                                   'timestamptz',
                                                                   'timetz'],
                                                      'globaldict256': [],
                                                      'globaldict4B': [],
                                                      'globaldict64k': [],
                                                      'lzo': ['bpchar',
                                                              'char',
                                                              'date',
                                                              'int2',
                                                              'int4',
                                                              'int8',
                                                              'numeric',
                                                              'super',
                                                              'text',
                                                              'time',
                                                              'timestamp',
                                                              'timestamptz',
                                                              'timetz',
                                                              'varchar'],
                                                      'mostly16': ['int4',
                                                                   'int8',
                                                                   'numeric'],
                                                      'mostly32': ['int8',
                                                                   'numeric'],
                                                      'mostly8': ['int2',
                                                                  'int4',
                                                                  'int8',
                                                                  'numeric'],
                                                      'raw': ['bool',
                                                              'bpchar',
                                                              'char',
                                                              'date',
                                                              'float4',
                                                              'float8',
                                                              'geometry',
                                                              'hllsketch',
                                                              'int2',
                                                              'int4',
                                                              'int8',
                                                              'numeric',
                                                              'super',
                                                              'text',
                                                              'time',
                                                              'timestamp',
                                                              'timestamptz',
                                                              'timetz',
                                                              'varchar'],
                                                      'runlength': ['bool',
                                                                    'bpchar',
                                                                    'char',
                                                                    'date',
                                                                    'float4',
                                                                    'float8',
                                                                    'int2',
                                                                    'int4',
                                                                    'int8',
                                                                    'numeric',
                                                                    'text',
                                                                    'time',
                                                                    'timestamp',
                                                                    'timestamptz',
                                                                    'timetz',
                                                                    'varchar'],
                                                      'text255': ['varchar'],
                                                      'text32k': ['text',
                                                                  'varchar'],
                                                      'zstd': ['bool',
                                                               'bpchar',
                                                               'char',
                                                               'date',
                                                               'float4',
                                                               'float8',
                                                               'int2',
                                                               'int4',
                                                               'int8',
                                                               'numeric',
                                                               'super',
                                                               'text',
                                                               'time',
                                                               'timestamp',
                                                               'timestamptz',
                                                               'timetz',
                                                               'varchar']},
                              'enumerated_encodings': ['INFO:  0,none',
                                                       'INFO:  1,bytedict',
                                                       'INFO:  2,delta',
                                                       'INFO:  3,lzo',
                                                       'INFO:  4,runlength',
                                                       'INFO:  5,delta32k',
                                                       'INFO:  7,text255',
                                                       'INFO:  '
                                                       '11,globaldict256',
                                                       'INFO:  '
                                                       '12,globaldict64k',
                                                       'INFO:  13,globaldict4B',
                                                       'INFO:  15,mostly8',
                                                       'INFO:  16,mostly16',
                                                       'INFO:  17,mostly32',
                                                       'INFO:  18,text32k',
                                                       'INFO:  19,zstd',
                                                       'INFO:  20,az64',
                                                       'INFO:  128,none',
                                                       'INFO:  131,lzo',
                                                       'INFO:  133,delta32k',
                                                       'INFO:  147,zstd',
                                                       'INFO:  148,az64']}}},
 'tests': {'dc2.large': {2: {}}},
 'versions': {'dc2.large': {2: 'PostgreSQL 8.0.2 on i686-pc-linux-gnu, '
                               'compiled by GCC gcc (GCC) 3.4.2 20041017 (Red '
                               'Hat 3.4.2-6.fc3), Redshift 1.0.32574'}}}

Redshift Observatory Slack

I’ve started up a Redshift Slack.

Join URL is;

https://join.slack.com/t/redshiftobservatory/shared_invite/zt-2vm3deqis-hc6h4GMDcG6Gs7~IECQNuQ