{"id":12215,"date":"2019-01-27T10:50:30","date_gmt":"2019-01-27T09:50:30","guid":{"rendered":"https:\/\/www.dbi-services.com\/blog\/postgresql-when-wal_level-to-logical\/"},"modified":"2023-06-15T16:33:25","modified_gmt":"2023-06-15T14:33:25","slug":"postgresql-when-wal_level-to-logical","status":"publish","type":"post","link":"https:\/\/www.dbi-services.com\/blog\/postgresql-when-wal_level-to-logical\/","title":{"rendered":"PostgreSQL: When wal_level to logical"},"content":{"rendered":"

By Mouhamadou Diaw<\/strong><\/p>\n

wal_level determines the quantity of information written to the WAL. With PostgreSQL 11 the parameter wal_level can have 3 values:
\n-minimal : only information needed to recover from a crash or an immediate shutdown
\n-replica : enough data to support WAL archiving and replication
\n-logical : enough information to support logical decoding<\/strong>.<\/p>\n

If we want to use logical decoding, wal_level should be set to logical. Logical decoding is the process of extracting all persistent changes to a database’s tables into a coherent, easy to understand format which can be interpreted without detailed knowledge of the database’s internal state.
\nIn PostgreSQL, logical decoding is implemented by decoding the contents of the write-ahead log, which describe changes on a storage level, into an application-specific form such as a stream of tuples or SQL statements.<\/p>\n

In this blog we are going to see some easy examples which will allow us to better understand this concept.<\/p>\n

Before we can use logical decoding the parameter wal_level should be set to logical. As we will create replications slots, the parameter max_replication_slots should also be at least 1.
\nBelow our values for these parameters<\/p>\n

\n
\n\n\n\n
\n
1<\/div>\n
2<\/div>\n
3<\/div>\n
4<\/div>\n
5<\/div>\n
6<\/div>\n
7<\/div>\n
8<\/div>\n
9<\/div>\n
10<\/div>\n
11<\/div>\n
12<\/div>\n
13<\/div>\n<\/td>\n
\n
\n
postgres=# show max_replication_slots ;<\/code><\/div>\n
\u00a0<\/code>max_replication_slots<\/code><\/div>\n
-----------------------<\/code><\/div>\n
\u00a0<\/code>10<\/code><\/div>\n
(1 row)<\/code><\/div>\n
<\/div>\n
postgres=# show wal_level ;<\/code><\/div>\n
\u00a0<\/code>wal_level<\/code><\/div>\n
-----------<\/code><\/div>\n
\u00a0<\/code>logical<\/code><\/div>\n
(1 row)<\/code><\/div>\n
<\/div>\n
postgres=#<\/code><\/div>\n<\/div>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<\/div>\n

First let\u2019s create a slot. For this we will use the function pg_create_logical_replication_slot()<\/p>\n

\n
\n\n\n\n
\n
1<\/div>\n
2<\/div>\n
3<\/div>\n
4<\/div>\n
5<\/div>\n
6<\/div>\n
7<\/div>\n<\/td>\n
\n
\n
postgres=# <\/code>SELECT<\/code> * <\/code>FROM<\/code> pg_create_logical_replication_slot(<\/code>'my_slot'<\/code>, <\/code>'test_decoding'<\/code>);<\/code><\/div>\n
\u00a0<\/code>slot_name |\u00a0\u00a0\u00a0 lsn<\/code><\/div>\n
-----------+-----------<\/code><\/div>\n
\u00a0<\/code>my_slot\u00a0\u00a0 | 0\/702B658<\/code><\/div>\n
(1 row)<\/code><\/div>\n
<\/div>\n
postgres=#<\/code><\/div>\n<\/div>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<\/div>\n

To inspect the changes at WAL level we can use the function pg_logical_slot_get_changes(). So let\u2019s call this function<\/p>\n

\n
\n\n\n\n
\n
1<\/div>\n
2<\/div>\n
3<\/div>\n
4<\/div>\n
5<\/div>\n
6<\/div>\n<\/td>\n
\n
\n
postgres=# <\/code>SELECT<\/code> * <\/code>FROM<\/code> pg_logical_slot_get_changes(<\/code>'my_slot'<\/code>, <\/code>NULL<\/code>, <\/code>NULL<\/code>);<\/code><\/div>\n
\u00a0<\/code>lsn | xid | data<\/code><\/div>\n
-----+-----+------<\/code><\/div>\n
(0 <\/code>rows<\/code>)<\/code><\/div>\n
<\/div>\n
postgres=#<\/code><\/div>\n<\/div>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<\/div>\n

This above output is expected because there is no change yet in our database
\nNow let\u2019s do some insert in the database and let\u2019s call again the function pg_logical_slot_get_changes()<\/p>\n

\n
\n\n\n\n
\n
1<\/div>\n
2<\/div>\n
3<\/div>\n
4<\/div>\n
5<\/div>\n
6<\/div>\n
7<\/div>\n
8<\/div>\n
9<\/div>\n
10<\/div>\n
11<\/div>\n
12<\/div>\n
13<\/div>\n
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15<\/div>\n
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17<\/div>\n
18<\/div>\n
19<\/div>\n<\/td>\n
\n
\n
postgres=# <\/code>begin<\/code>;<\/code><\/div>\n
BEGIN<\/code><\/div>\n
postgres=# <\/code>insert<\/code> into<\/code> mytab <\/code>values<\/code> (1,<\/code>'t1'<\/code>);<\/code><\/div>\n
INSERT<\/code> 0 1<\/code><\/div>\n
postgres=# <\/code>insert<\/code> into<\/code> mytab <\/code>values<\/code> (2,<\/code>'t2'<\/code>);<\/code><\/div>\n
INSERT<\/code> 0 1<\/code><\/div>\n
postgres=# <\/code>commit<\/code>;<\/code><\/div>\n
COMMIT<\/code><\/div>\n
<\/div>\n
postgres=# <\/code>SELECT<\/code> * <\/code>FROM<\/code> pg_logical_slot_get_changes(<\/code>'my_slot'<\/code>, <\/code>NULL<\/code>, <\/code>NULL<\/code>);<\/code><\/div>\n
\u00a0\u00a0\u00a0\u00a0<\/code>lsn\u00a0\u00a0\u00a0 | xid |\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 data<\/code><\/div>\n
-----------+-----+------------------------------------------------------------------------<\/code><\/div>\n
\u00a0<\/code>0\/703F538 | 582 | <\/code>BEGIN<\/code> 582<\/code><\/div>\n
\u00a0<\/code>0\/703F538 | 582 | <\/code>table<\/code> public<\/code>.mytab: <\/code>INSERT<\/code>: id[<\/code>integer<\/code>]:1 <\/code>name<\/code>[<\/code>character<\/code> varying<\/code>]:<\/code>'t1'<\/code><\/div>\n
\u00a0<\/code>0\/703F5B0 | 582 | <\/code>table<\/code> public<\/code>.mytab: <\/code>INSERT<\/code>: id[<\/code>integer<\/code>]:2 <\/code>name<\/code>[<\/code>character<\/code> varying<\/code>]:<\/code>'t2'<\/code><\/div>\n
\u00a0<\/code>0\/703F620 | 582 | <\/code>COMMIT<\/code> 582<\/code><\/div>\n
(4 <\/code>rows<\/code>)<\/code><\/div>\n
<\/div>\n
postgres=#<\/code><\/div>\n<\/div>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<\/div>\n

As expected we can see changes that were made.
\nNow what happen if we call again the same function?<\/p>\n

\n
\n\n\n\n
\n
1<\/div>\n
2<\/div>\n
3<\/div>\n
4<\/div>\n
5<\/div>\n
6<\/div>\n<\/td>\n
\n
\n
postgres=# <\/code>SELECT<\/code> * <\/code>FROM<\/code> pg_logical_slot_get_changes(<\/code>'my_slot'<\/code>, <\/code>NULL<\/code>, <\/code>NULL<\/code>);<\/code><\/div>\n
\u00a0<\/code>lsn | xid | data<\/code><\/div>\n
-----+-----+------<\/code><\/div>\n
(0 <\/code>rows<\/code>)<\/code><\/div>\n
<\/div>\n
postgres=#<\/code><\/div>\n<\/div>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<\/div>\n

The changes are no longer reported. It\u2019s normal because with the function pg_logical_slot_get_changes(), changes are consumed (will not be returned again). If we want the changes not to be consumed we can use the function pg_logical_slot_peek_changes(). This function behaves like the first one, except that changes are not consumed; that is, they will be returned again on future calls.<\/p>\n

\n
\n\n\n\n
\n
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35<\/div>\n
36<\/div>\n<\/td>\n
\n
\n
postgres=# <\/code>begin<\/code>;<\/code><\/div>\n
BEGIN<\/code><\/div>\n
postgres=# <\/code>insert<\/code> into<\/code> mytab <\/code>values<\/code> (3,<\/code>'t3'<\/code>);<\/code><\/div>\n
INSERT<\/code> 0 1<\/code><\/div>\n
postgres=# <\/code>insert<\/code> into<\/code> mytab <\/code>values<\/code> (4,<\/code>'t4'<\/code>);<\/code><\/div>\n
INSERT<\/code> 0 1<\/code><\/div>\n
postgres=# <\/code>commit<\/code>;<\/code><\/div>\n
COMMIT<\/code><\/div>\n
postgres=# <\/code>delete<\/code> from<\/code> mytab <\/code>where<\/code> id=1;<\/code><\/div>\n
DELETE<\/code> 1<\/code><\/div>\n
<\/div>\n
postgres=# <\/code>SELECT<\/code> * <\/code>FROM<\/code> pg_logical_slot_peek_changes(<\/code>'my_slot'<\/code>, <\/code>NULL<\/code>, <\/code>NULL<\/code>);<\/code><\/div>\n
\u00a0\u00a0\u00a0\u00a0<\/code>lsn\u00a0\u00a0\u00a0 | xid |\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 data<\/code><\/div>\n
-----------+-----+------------------------------------------------------------------------<\/code><\/div>\n
\u00a0<\/code>0\/703F738 | 583 | <\/code>BEGIN<\/code> 583<\/code><\/div>\n
\u00a0<\/code>0\/703F738 | 583 | <\/code>table<\/code> public<\/code>.mytab: <\/code>INSERT<\/code>: id[<\/code>integer<\/code>]:3 <\/code>name<\/code>[<\/code>character<\/code> varying<\/code>]:<\/code>'t3'<\/code><\/div>\n
\u00a0<\/code>0\/703F838 | 583 | <\/code>table<\/code> public<\/code>.mytab: <\/code>INSERT<\/code>: id[<\/code>integer<\/code>]:4 <\/code>name<\/code>[<\/code>character<\/code> varying<\/code>]:<\/code>'t4'<\/code><\/div>\n
\u00a0<\/code>0\/703F8A8 | 583 | <\/code>COMMIT<\/code> 583<\/code><\/div>\n
\u00a0<\/code>0\/703F8E0 | 584 | <\/code>BEGIN<\/code> 584<\/code><\/div>\n
\u00a0<\/code>0\/703F8E0 | 584 | <\/code>table<\/code> public<\/code>.mytab: <\/code>DELETE<\/code>: (<\/code>no<\/code>-tuple-data)<\/code><\/div>\n
\u00a0<\/code>0\/703F948 | 584 | <\/code>COMMIT<\/code> 584<\/code><\/div>\n
(7 <\/code>rows<\/code>)<\/code><\/div>\n
<\/div>\n
postgres=# <\/code>SELECT<\/code> * <\/code>FROM<\/code> pg_logical_slot_peek_changes(<\/code>'my_slot'<\/code>, <\/code>NULL<\/code>, <\/code>NULL<\/code>);<\/code><\/div>\n
\u00a0\u00a0\u00a0\u00a0<\/code>lsn\u00a0\u00a0\u00a0 | xid |\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 data<\/code><\/div>\n
-----------+-----+------------------------------------------------------------------------<\/code><\/div>\n
\u00a0<\/code>0\/703F738 | 583 | <\/code>BEGIN<\/code> 583<\/code><\/div>\n
\u00a0<\/code>0\/703F738 | 583 | <\/code>table<\/code> public<\/code>.mytab: <\/code>INSERT<\/code>: id[<\/code>integer<\/code>]:3 <\/code>name<\/code>[<\/code>character<\/code> varying<\/code>]:<\/code>'t3'<\/code><\/div>\n
\u00a0<\/code>0\/703F838 | 583 | <\/code>table<\/code> public<\/code>.mytab: <\/code>INSERT<\/code>: id[<\/code>integer<\/code>]:4 <\/code>name<\/code>[<\/code>character<\/code> varying<\/code>]:<\/code>'t4'<\/code><\/div>\n
\u00a0<\/code>0\/703F8A8 | 583 | <\/code>COMMIT<\/code> 583<\/code><\/div>\n
\u00a0<\/code>0\/703F8E0 | 584 | <\/code>BEGIN<\/code> 584<\/code><\/div>\n
\u00a0<\/code>0\/703F8E0 | 584 | <\/code>table<\/code> public<\/code>.mytab: <\/code>DELETE<\/code>: (<\/code>no<\/code>-tuple-data)<\/code><\/div>\n
\u00a0<\/code>0\/703F948 | 584 | <\/code>COMMIT<\/code> 584<\/code><\/div>\n
(7 <\/code>rows<\/code>)<\/code><\/div>\n
<\/div>\n
postgres=#<\/code><\/div>\n<\/div>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<\/div>\n

Logical decoding can also be managed using pg_recvlogical included in the PostgreSQL distribution.
\nLet\u2019s create a slot using pg_recvlogical<\/p>\n

\n
\n\n\n\n
\n
1<\/div>\n<\/td>\n
\n
\n
[postgres@dbi-pg-essentials_3 PG1]$ pg_recvlogical -d postgres <\/code>--slot=myslot_2\u00a0 --create-slot<\/code><\/div>\n<\/div>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<\/div>\n

And let\u2019s start the streaming in a first terminal<\/p>\n

\n
\n\n\n\n
\n
1<\/div>\n<\/td>\n
\n
\n
[postgres@dbi-pg-essentials_3 PG1]$ pg_recvlogical -d postgres <\/code>--slot=myslot_2\u00a0 --start -f -<\/code><\/div>\n<\/div>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<\/div>\n

If we do an insert in the database from a second terminal<\/p>\n

\n
\n\n\n\n
\n
1<\/div>\n
2<\/div>\n
3<\/div>\n<\/td>\n
\n
\n
postgres=# <\/code>insert<\/code> into<\/code> mytab <\/code>values<\/code> (9,<\/code>'t9'<\/code>);<\/code><\/div>\n
INSERT<\/code> 0 1<\/code><\/div>\n
postgres=#<\/code><\/div>\n<\/div>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<\/div>\n

We will see following in the first terminal<\/p>\n

\n
\n\n\n\n
\n
1<\/div>\n
2<\/div>\n
3<\/div>\n
4<\/div>\n<\/td>\n
\n
\n
[postgres@dbi-pg-essentials_3 PG1]$ pg_recvlogical -d postgres <\/code>--slot=myslot_2\u00a0 --start -f -\u00a0\u00a0 <\/code><\/div>\n
BEGIN<\/code> 587<\/code><\/div>\n
table<\/code> public<\/code>.mytab: <\/code>INSERT<\/code>: id[<\/code>integer<\/code>]:9 <\/code>name<\/code>[<\/code>character<\/code> varying<\/code>]:<\/code>'t9'<\/code><\/div>\n
COMMIT<\/code> 587<\/code><\/div>\n<\/div>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<\/div>\n

Conclusion<\/h3>\n

In this blog we have seen that if we want to do logical decoding, we have to set the parameter wal_level to logical. Be aware that setting wal_level to logical can increase the volume of generated WAL. If we just want replication or archiving WALs, the value replica is enough with PostgreSQL.<\/p>\n","protected":false},"excerpt":{"rendered":"

By Mouhamadou Diaw wal_level determines the quantity of information written to the WAL. With PostgreSQL 11 the parameter wal_level can have 3 values: -minimal : only information needed to recover from a crash or an immediate shutdown -replica : enough data to support WAL archiving and replication -logical : enough information to support logical decoding. […]<\/p>\n","protected":false},"author":27,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[229,83],"tags":[1555,77,1556,1557],"type_dbi":[],"class_list":["post-12215","post","type-post","status-publish","format-standard","hentry","category-database-administration-monitoring","category-postgresql","tag-logical-decoding","tag-postgresql","tag-replication-slot","tag-wal_level"],"acf":[],"yoast_head":"\nPostgreSQL: When wal_level to logical - dbi Blog<\/title>\n<meta name=\"description\" content=\"PostgreSQL , wal_level, logical decoding,replication slot\" \/>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/www.dbi-services.com\/blog\/postgresql-when-wal_level-to-logical\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"PostgreSQL: When wal_level to logical\" \/>\n<meta property=\"og:description\" content=\"PostgreSQL , wal_level, logical decoding,replication slot\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.dbi-services.com\/blog\/postgresql-when-wal_level-to-logical\/\" \/>\n<meta property=\"og:site_name\" content=\"dbi Blog\" \/>\n<meta property=\"article:published_time\" content=\"2019-01-27T09:50:30+00:00\" \/>\n<meta property=\"article:modified_time\" content=\"2023-06-15T14:33:25+00:00\" \/>\n<meta name=\"author\" content=\"Oracle Team\" \/>\n<meta name=\"twitter:card\" content=\"summary_large_image\" \/>\n<meta name=\"twitter:label1\" content=\"Written by\" \/>\n\t<meta name=\"twitter:data1\" content=\"Oracle Team\" \/>\n\t<meta name=\"twitter:label2\" content=\"Est. reading time\" \/>\n\t<meta name=\"twitter:data2\" content=\"4 minutes\" \/>\n<script type=\"application\/ld+json\" class=\"yoast-schema-graph\">{\"@context\":\"https:\/\/schema.org\",\"@graph\":[{\"@type\":\"Article\",\"@id\":\"https:\/\/www.dbi-services.com\/blog\/postgresql-when-wal_level-to-logical\/#article\",\"isPartOf\":{\"@id\":\"https:\/\/www.dbi-services.com\/blog\/postgresql-when-wal_level-to-logical\/\"},\"author\":{\"name\":\"Oracle Team\",\"@id\":\"https:\/\/www.dbi-services.com\/blog\/#\/schema\/person\/66ab87129f2d357f09971bc7936a77ee\"},\"headline\":\"PostgreSQL: When wal_level to logical\",\"datePublished\":\"2019-01-27T09:50:30+00:00\",\"dateModified\":\"2023-06-15T14:33:25+00:00\",\"mainEntityOfPage\":{\"@id\":\"https:\/\/www.dbi-services.com\/blog\/postgresql-when-wal_level-to-logical\/\"},\"wordCount\":468,\"commentCount\":0,\"keywords\":[\"logical decoding\",\"PostgreSQL\",\"replication slot\",\"wal_level\"],\"articleSection\":[\"Database Administration & Monitoring\",\"PostgreSQL\"],\"inLanguage\":\"en-US\",\"potentialAction\":[{\"@type\":\"CommentAction\",\"name\":\"Comment\",\"target\":[\"https:\/\/www.dbi-services.com\/blog\/postgresql-when-wal_level-to-logical\/#respond\"]}]},{\"@type\":\"WebPage\",\"@id\":\"https:\/\/www.dbi-services.com\/blog\/postgresql-when-wal_level-to-logical\/\",\"url\":\"https:\/\/www.dbi-services.com\/blog\/postgresql-when-wal_level-to-logical\/\",\"name\":\"PostgreSQL: When wal_level to logical - 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