Adds a section about `change_column_default`

What does this MR do?

Includes a new section under our migration style guide for the usage of change_column_default. Particularly, specifying that it's not a risky operation for larger tables and can be used in a single transaction

Related issues

Author's checklist

• Follow the Documentation Guidelines and Style Guide.
• [-] If applicable, update the permissions table.
• Link docs to and from the higher-level index page, plus other related docs where helpful.
• Apply the documentation label.

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changed milestone to %12.4

added database docs-only documentation labels

added databasereview pending label

	1 Warning
⚠	Please add a throughput label to this merge request.

	2 Messages
📖	This merge request adds or changes files that require a review from the Database team.
📖	CHANGELOG missing: If this merge request doesn’t need a CHANGELOG entry, feel free to ignore this message. You can create one with: bin/changelog -m 18090 "Adds a section about `change_column_default`" If you want to create a changelog entry for GitLab EE, run the following instead: bin/changelog --ee -m 18090 "Adds a section about `change_column_default`" Note: Merge requests with ~backstage, ci-build, meta do not trigger this check.

This merge request requires a database review. To make sure these changes are reviewed, take the following steps:

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Generated by 🚫 Danger

@abrandl could you please review? I'm trying to capture the past conversations on the DB hours.

PS. This is the last one I swear 🙈

assigned to @abrandl and unassigned @mayra-cabrera

added 1 commit

• 5a38a6ed - Add a section about change_column_default

Compare with previous version

approved this merge request

PS. This is the last one I swear

No, this is really great, thank you! 😄

@mayra-cabrera I left a comment as a potential addition, but feel free to merge as is, too.

assigned to @mayra-cabrera and unassigned @abrandl

added databaseapproved label and removed databasereview pending label

added 104 commits

• 5a38a6ed...3630d71d - 103 commits from branch master
• a5ef00dd - Add a section about change_column_default

Compare with previous version

marked the checklist item Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes. as completed

marked the checklist item Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review. as completed

@axil could you please review?

assigned to @axil and unassigned @mayra-cabrera

resolved all threads

merged

mentioned in commit e5350659

resolved all threads

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added workflowstaging label

This merge request has been deployed to the GitLab.com environment gprd-cny in GitLab auto-deploy version 12.4.201910130810-44adee04c32.30ae0ff6d6f.

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added workflowcanary label and removed workflowstaging label

This merge request has been deployed to the GitLab.com environment gprd in GitLab auto-deploy version 12.4.201910160011-08e6905db3d.4748e740ecf.

A list of all the deployed commits can be found here.

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added workflowproduction label and removed workflowcanary label

mentioned in commit Dalahro/gitlab@4d1a3d31

added typemaintenance label