Postgres and subquery support

[amoffat]

Description

Support for the Postgres database and also support for subqueries and CTEs

Confirmations

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Code quality

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• My commits are descriptive and self-contained
• I have written docstrings for any new code
• All non-obvious additions and changes have been clearly commented
• I have added test coverage for any code changes
• I have added or updated the /docs where applicable

Compatibility

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• These changes do not affect any behavior or interfaces (no version change)
• These changes fix intended behavior or interfaces (patch/micro version change)
• These changes add new behavior or interfaces (minor version change)
• These changes alter existing behavior or interfaces (major version change)

Contribution rationale

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• This contribution is a bug fix
• This contribution is a documentation fix
• This contribution is part of the public roadmap
• This contribution was communicated in advance and approved by the author

Licensing

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• I am contributing as individual and I have signed the Individual (CLA)
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[amoffat]

comments

[amoffat]

changelog date, then good

[amoffat]

good