Re: Perform streaming logical transactions by background workers and parallel apply

Here are my review comments for patch v86-0001.

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General

1.

IIUC the GUC name was made generic 'logical_replication_mode' so that
multiple developer GUCs are not needed later.

But IMO those current option values (buffered/immediate) for that GUC
are maybe a bit too generic. Perhaps in future, we might want more
granular control than that allows. e.g. I can imagine there might be
multiple different meanings for what "buffered" means. If there is any
chance of the generic values being problematic later then maybe they
should be made more specific up-front.

e.g. maybe like:
logical_replication_mode = buffered_decoding
logical_replication_mode = immediate_decoding

Thoughts?

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Commit message

2.
Since we may extend the developer option logical_decoding_mode to to test the
parallel apply of large transaction on subscriber, rename this option to
logical_replication_mode to make it easier to understand.

~

2a
typo "to to"

typo "large transaction on subscriber" --> "large transactions on the
subscriber"

~

2b.
IMO better to rephrase the whole paragraph like shown below.

SUGGESTION

Rename the developer option 'logical_decoding_mode' to the more
flexible name 'logical_replication_mode' because doing so will make it
easier to extend this option in future to help test other areas of
logical replication.

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doc/src/sgml/config.sgml

3.
Allows streaming or serializing changes immediately in logical
decoding. The allowed values of logical_replication_mode are buffered
and immediate. When set to immediate, stream each change if streaming
option (see optional parameters set by CREATE SUBSCRIPTION) is
enabled, otherwise, serialize each change. When set to buffered, which
is the default, decoding will stream or serialize changes when
logical_decoding_work_mem is reached.

~

IMO it's more clear to say the default when the options are first
mentioned. So I suggested removing the "which is the default" part,
and instead saying:

BEFORE
The allowed values of logical_replication_mode are buffered and immediate.

AFTER
The allowed values of logical_replication_mode are buffered and
immediate. The default is buffered.

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src/backend/utils/misc/guc_tables.c

4.
@@ -396,8 +396,8 @@ static const struct config_enum_entry
ssl_protocol_versions_info[] = {
};

static const struct config_enum_entry logical_decoding_mode_options[] = {
- {"buffered", LOGICAL_DECODING_MODE_BUFFERED, false},
- {"immediate", LOGICAL_DECODING_MODE_IMMEDIATE, false},
+ {"buffered", LOGICAL_REP_MODE_BUFFERED, false},
+ {"immediate", LOGICAL_REP_MODE_IMMEDIATE, false},
{NULL, 0, false}
};

I noticed this array is still called "logical_decoding_mode_options".
Was that deliberate?

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Kind Regards,
Peter Smith.
Fujitsu Australia