Support for multiple schemas in a database

[zachmu]

Companion PRs:

dolthub/vitess#338
dolthub/go-mysql-server#2466
dolthub/doltgresql#181

This PR is a proof of concept for addressing multiple table namespaces (schemas) in a database. This is to support schemas in Doltgres initially, but could also be used to implement storing multiple databases in the same Dolt commit graph if we wanted to do that. All tables are stored in the root's table map like always, but tables with non-empty schemas have that schema name prepended in its map key, surrounded by null bytes.

There are lots of things that don't work yet. This is what does work:

create schema mySchema;
create schema otherSchema;
CREATE TABLE mySchema.test (pk BIGINT PRIMARY KEY, v1 BIGINT);
insert into mySchema.test values (1,1), (2,2);
CREATE TABLE otherSchema.test (pk BIGINT PRIMARY KEY, v1 BIGINT);
insert into otherSchema.test values (3,3), (4,4);
SELECT * FROM mySchema.test;
SELECT * FROM otherSchema.test;

Things I specifically want feedback on:

• I am not crazy about the sweeping changes to table-access interfaces in root_val.go. In particular it seems likely to cause errors over time as people forget to include schema names in code changes where necessary, silently breaking doltgres. Another option I considered was to introduce some sort of RootReader interface on top of root value, which you create with a particular schema name, and thereafter all table accesses reference the given schema. There may be places where we have to juggle multiple RootReader instances at once, rather than just asking for qualified table names from a single root. But this is the pattern I used for the boundary between GMS and Dolt and it worked well, so maybe it could work for root_val.go as well.

[zachmu]

Regarding a marker approach, we already disallow all tables from the "dolt_" namespace right? Perhaps we could make use of that in some way to form a marker.

This is the route I went with, but instead of dolt_ I used null bytes to offset the schema name.

[Hydrocharged]

Overall looks good to me!

[coffeegoddd]

@zachmu DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
2ecfd80	ok	5937457

version	total_tests
2ecfd80	5937457

correctness_percentage
100.0

[coffeegoddd]

@zachmu DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
053d1f2	ok	5937457

version	total_tests
053d1f2	5937457

correctness_percentage
100.0

[coffeegoddd]

@zachmu DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
027054e	ok	5937457

version	total_tests
027054e	5937457

correctness_percentage
100.0

[coffeegoddd]

@coffeegoddd DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
1da1f41	ok	5937457

version	total_tests
1da1f41	5937457

correctness_percentage
100.0

[coffeegoddd]

@zachmu DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
4ed8a62	ok	5937457

version	total_tests
4ed8a62	5937457

correctness_percentage
100.0

[coffeegoddd]

@coffeegoddd DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
fe845d8	ok	5937457

version	total_tests
fe845d8	5937457

correctness_percentage
100.0

[coffeegoddd]

@zachmu DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
cf8c356	ok	5937457

version	total_tests
cf8c356	5937457

correctness_percentage
100.0