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Rethink definition of pg_attribute.attcompression.

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Redefine '\0' (InvalidCompressionMethod) as meaning "if we need to
compress, use the current setting of default_toast_compression".
This allows '\0' to be a suitable default choice regardless of
datatype, greatly simplifying code paths that initialize tupledescs
and the like.  It seems like a more user-friendly approach as well,
because now the default compression choice doesn't migrate into table
definitions, meaning that changing default_toast_compression is
usually sufficient to flip an installation's behavior; one needn't
tediously issue per-column ALTER SET COMPRESSION commands.

Along the way, fix a few minor bugs and documentation issues
with the per-column-compression feature.  Adopt more robust
APIs for SetIndexStorageProperties and GetAttributeCompression.

Bump catversion because typical contents of attcompression will now
be different.  We could get away without doing that, but it seems
better to ensure v14 installations all agree on this.  (We already
forced initdb for beta2, anyway.)

Discussion: https://postgr.es/m/626613.1621787110@sss.pgh.pa.us
This commit is contained in:
Tom Lane
2021-05-27 13:24:24 -04:00
parent a717e5c771
commit e6241d8e03
29 changed files with 261 additions and 384 deletions
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12
doc/src/sgml/catalogs.sgml
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@ -1261,10 +1261,14 @@
<structfield>attcompression</structfield> <type>char</type>
</para>
<para>
The current compression method of the column. If it is an invalid
compression method (<literal>'\0'</literal>) then column data will not
be compressed. Otherwise, <literal>'p'</literal> = pglz compression or
<literal>'l'</literal> = <productname>LZ4</productname> compression.
The current compression method of the column. Typically this is
<literal>'\0'</literal> to specify use of the current default setting
(see <xref linkend="guc-default-toast-compression"/>). Otherwise,
<literal>'p'</literal> selects pglz compression, while
<literal>'l'</literal> selects <productname>LZ4</productname>
compression. However, this field is ignored
whenever <structfield>attstorage</structfield> does not allow
compression.
</para></entry>
</row>

15
doc/src/sgml/config.sgml
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@ -8256,13 +8256,14 @@ COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
<para>
This variable sets the default
<link linkend="storage-toast">TOAST</link>
compression method for columns of newly-created tables. The
<command>CREATE TABLE</command> statement can override this default
by specifying the <literal>COMPRESSION</literal> column option.
The supported compression methods are <literal>pglz</literal> and,
if <productname>PostgreSQL</productname> was compiled with
<literal>--with-lz4</literal>, <literal>lz4</literal>.
compression method for values of compressible columns.
(This can be overridden for individual columns by setting
the <literal>COMPRESSION</literal> column option in
<command>CREATE TABLE</command> or
<command>ALTER TABLE</command>.)
The supported compression methods are <literal>pglz</literal> and
(if <productname>PostgreSQL</productname> was compiled with
<option>--with-lz4</option>) <literal>lz4</literal>.
The default is <literal>pglz</literal>.
</para>
</listitem>

4
doc/src/sgml/func.sgml
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@ -26253,10 +26253,10 @@ postgres=# SELECT * FROM pg_walfile_name_offset(pg_stop_backup());
<primary>pg_column_compression</primary>
</indexterm>
<function>pg_column_compression</function> ( <type>"any"</type> )
<returnvalue>integer</returnvalue>
<returnvalue>text</returnvalue>
</para>
<para>
Shows the compression algorithm that was used to compress a
Shows the compression algorithm that was used to compress
an individual variable-length value. Returns <literal>NULL</literal>
if the value is not compressed.
</para></entry>

30
doc/src/sgml/ref/alter_table.sgml
View File

@ -104,7 +104,6 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY [ ( <replaceable>sequence_options</replaceable> ) ] |
UNIQUE <replaceable class="parameter">index_parameters</replaceable> |
PRIMARY KEY <replaceable class="parameter">index_parameters</replaceable> |
COMPRESSION <replaceable class="parameter">compression_method</replaceable> |
REFERENCES <replaceable class="parameter">reftable</replaceable> [ ( <replaceable class="parameter">refcolumn</replaceable> ) ] [ MATCH FULL | MATCH PARTIAL | MATCH SIMPLE ]
[ ON DELETE <replaceable class="parameter">referential_action</replaceable> ] [ ON UPDATE <replaceable class="parameter">referential_action</replaceable> ] }
[ DEFERRABLE | NOT DEFERRABLE ] [ INITIALLY DEFERRED | INITIALLY IMMEDIATE ]
@ -391,24 +390,27 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
</term>
<listitem>
<para>
This sets the compression method to be used for data inserted into a column.
This form sets the compression method for a column, determining how
values inserted in future will be compressed (if the storage mode
permits compression at all).
This does not cause the table to be rewritten, so existing data may still
be compressed with other compression methods. If the table is rewritten with
<command>VACUUM FULL</command> or <command>CLUSTER</command>, or restored
with <application>pg_restore</application>, then all tuples are rewritten
with the configured compression methods.
Also, note that when data is inserted from another relation (for example,
by <command>INSERT ... SELECT</command>), tuples from the source data are
not necessarily detoasted, and any previously compressed data is retained
with its existing compression method, rather than recompressing with the
compression methods of the target columns.
with <application>pg_restore</application>, then all values are rewritten
with the configured compression method.
However, when data is inserted from another relation (for example,
by <command>INSERT ... SELECT</command>), values from the source table are
not necessarily detoasted, so any previously compressed data may retain
its existing compression method, rather than being recompressed with the
compression method of the target column.
The supported compression
methods are <literal>pglz</literal> and <literal>lz4</literal>.
<literal>lz4</literal> is available only if <literal>--with-lz4</literal>
was used when building <productname>PostgreSQL</productname>.
(<literal>lz4</literal> is available only if <option>--with-lz4</option>
was used when building <productname>PostgreSQL</productname>.) In
addition, <replaceable class="parameter">compression_method</replaceable>
can be <literal>default</literal>, which selects the default behavior of
consulting the <xref linkend="guc-default-toast-compression"/> setting
at the time of data insertion to determine the method to use.
</para>
</listitem>
</varlistentry>

25
doc/src/sgml/ref/create_table.sgml
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@ -22,7 +22,7 @@ PostgreSQL documentation
<refsynopsisdiv>
<synopsis>
CREATE [ [ GLOBAL | LOCAL ] { TEMPORARY | TEMP } | UNLOGGED ] TABLE [ IF NOT EXISTS ] <replaceable class="parameter">table_name</replaceable> ( [
{ <replaceable class="parameter">column_name</replaceable> <replaceable class="parameter">data_type</replaceable> [ COLLATE <replaceable>collation</replaceable> ] [ COMPRESSION <replaceable>compression_method</replaceable> ] [ <replaceable class="parameter">column_constraint</replaceable> [ ... ] ]
{ <replaceable class="parameter">column_name</replaceable> <replaceable class="parameter">data_type</replaceable> [ COMPRESSION <replaceable>compression_method</replaceable> ] [ COLLATE <replaceable>collation</replaceable> ] [ <replaceable class="parameter">column_constraint</replaceable> [ ... ] ]
| <replaceable>table_constraint</replaceable>
| LIKE <replaceable>source_table</replaceable> [ <replaceable>like_option</replaceable> ... ] }
[, ... ]
@ -293,17 +293,22 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
<listitem>
<para>
The <literal>COMPRESSION</literal> clause sets the compression method
for a column. Compression is supported only for variable-width data
types, and is used only for columns whose storage type is main or
extended. (See <xref linkend="sql-altertable"/> for information on
column storage types.) Setting this property for a partitioned table
for the column. Compression is supported only for variable-width data
types, and is used only when the column's storage mode
is <literal>main</literal> or <literal>extended</literal>.
(See <xref linkend="sql-altertable"/> for information on
column storage modes.) Setting this property for a partitioned table
has no direct effect, because such tables have no storage of their own,
but the configured value is inherited by newly-created partitions.
but the configured value will be inherited by newly-created partitions.
The supported compression methods are <literal>pglz</literal> and
<literal>lz4</literal>. <literal>lz4</literal> is available only if
<literal>--with-lz4</literal> was used when building
<productname>PostgreSQL</productname>. The default
is <literal>pglz</literal>.
<literal>lz4</literal>. (<literal>lz4</literal> is available only if
<option>--with-lz4</option> was used when building
<productname>PostgreSQL</productname>.) In addition,
<replaceable class="parameter">compression_method</replaceable>
can be <literal>default</literal> to explicitly specify the default
behavior, which is to consult the
<xref linkend="guc-default-toast-compression"/> setting at the time of
data insertion to determine the method to use.
</para>
</listitem>
</varlistentry>

4
doc/src/sgml/ref/pg_dump.sgml
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@ -975,8 +975,8 @@ PostgreSQL documentation
<para>
Do not output commands to set <acronym>TOAST</acronym> compression
methods.
With this option, all objects will be created using whichever
compression method is the default during restore.
With this option, all columns will be restored with the default
compression setting.
</para>
</listitem>
</varlistentry>

6
doc/src/sgml/ref/pg_dumpall.sgml
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@ -464,12 +464,12 @@ PostgreSQL documentation
<para>
Do not output commands to set <acronym>TOAST</acronym> compression
methods.
With this option, all objects will be created using whichever
compression method is the default during restore.
With this option, all columns will be restored with the default
compression setting.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>--no-unlogged-table-data</option></term>
<listitem>

17
doc/src/sgml/storage.sgml
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@ -376,6 +376,16 @@ but the varlena header does not tell whether it has occurred &mdash;
the content of the <acronym>TOAST</acronym> pointer tells that, instead.
</para>
<para>
The compression technique used for either in-line or out-of-line compressed
data can be selected for each column by setting
the <literal>COMPRESSION</literal> column option in <command>CREATE
TABLE</command> or <command>ALTER TABLE</command>. The default for columns
with no explicit setting is to consult the
<xref linkend="guc-default-toast-compression"/> parameter at the time data is
inserted.
</para>
<para>
As mentioned, there are multiple types of <acronym>TOAST</acronym> pointer datums.
The oldest and most common type is a pointer to out-of-line data stored in
@ -392,13 +402,6 @@ useful for avoiding copying and redundant processing of large data values.
Further details appear in <xref linkend="storage-toast-inmemory"/>.
</para>
<para>
The compression technique used for either in-line or out-of-line compressed
data can be selected using the <literal>COMPRESSION</literal> option on a per-column
basis when creating a table. The default for columns with no explicit setting
is taken from the value of <xref linkend="guc-default-toast-compression" />.
</para>
<sect2 id="storage-toast-ondisk">
<title>Out-of-Line, On-Disk TOAST Storage</title>