Use postgres-specific tables to read database schema

[michalbundyra]

Fixes performance issues noted in #199.

On my production server, the original query took 22 seconds to retrieve 272 rows. This new query takes 204ms and retrieves the same rows.

This is based off @alextech query in #199 and adapted to be a drop-in replacement (it returns the correct values for check_option and is_updatable.

Closes #199

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Originally posted by @mat128 at zendframework/zend-db#321

[michalbundyra]

@mat128 thank for the PR, it looks very good! We need to provide a test for this change. We have a test/integration folder where we can test PostgreSQL using Travis CI, see this database test configuration.
Have a look and let me know if you need assistance adding this integration test, thanks!

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Originally posted by @ezimuel at zendframework/zend-db#321 (comment)

[weierophinney]

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If you are looking for an actively maintained package alternative, we recommend:

• doctrine/dbal