fix: Avoid creating MoveTables in case of non-empty target tables

[beingnoble03]

Description

This PR adds a validation check for empty tables in the target keyspace while creating MoveTables workflow.

Related Issue(s)

• MoveTables Create: ensure that none of the tables to be moved have data  #16817

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Checklist

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[mattlord]

Why do this for every shard? Can't we build the query once and then use it on each shard?

Keep in mind that the size of this query is unbounded, so we could hit max_allowed_packet. I want to say that there's also a limit on the number of UNIONs you can do in a single statement but I didn't find any docs on that in a quick search. For JOINs e.g.:

The maximum number of tables that can be referenced in a single join is 61. This includes a join handled by merging derived tables and views in the FROM clause into the outer query block (see [Section 10.2.2.4, “Optimizing Derived Tables, View References, and Common Table Expressions with Merging or Materialization”](https://dev.mysql.com/doc/refman/8.4/en/derived-table-optimization.html)).

So that may be what we bump up against here.

[beingnoble03]

Thanks Matt for finding this. Fixed that. But even I couldn't find out the limit of UNIONs on any docs or web search. What do you think should be the limit? Should we break the query down into multiple queries?

[beingnoble03]

This was probably a mistake.

[codecov]

Codecov Report

Attention: Patch coverage is 87.30159% with 8 lines in your changes missing coverage. Please review.

Project coverage is 69.45%. Comparing base (95f2e3e) to head (8393dea).
Report is 63 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtctl/workflow/utils.go	85.71%	6 Missing ⚠️
go/vt/vtctl/workflow/materializer.go	90.47%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16826      +/-   ##
==========================================
- Coverage   69.51%   69.45%   -0.06%     
==========================================
  Files        1569     1571       +2     
  Lines      202517   203121     +604     
==========================================
+ Hits       140780   141083     +303     
- Misses      61737    62038     +301     

☔ View full report in Codecov by Sentry.
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[beingnoble03]

I am working on the failing TestVtctlMigrate e2e test.

[mattlord]

You can instead use maps.Keys(hasTargetTable) with validateEmptyTables.

[mattlord]

I think we need a way to handle cases where there are many tables as the number of tables is unbounded. Two factors we'll have to consider:

1. This one statement could be so long that it is beyond max_allowed_packet
2. This one statement could affect more than the maximum allowed tables

In either case, the user is stuck and there is no outlet. At least unless we add a new flag that skips this test / allows for tables with existing data OR splitting the workflow up (which itself might be a bit daunting to manually specify hundreds or thousands of tables).

[rohit-nayak-ps]

There is one issue already in the existing code: We get the list of tables just for one shard in getTablesInKeyspace() However each target shard might have a different set of tables which exist. If the selected shard has the table, but other shards not, then we will end up not creating the target tables in some shards.

This extends to the new check we are adding. If a table is present in the first code then we will also check for data in the table in other shards. If the other shards don't have that table then the check for data will fail with a "table not found"

In addition, the getSchema() call, made while trying to getTablesInKeyspace() that we make is a heavy duty query, just to get the list of tables.

We may want to consider a different approach that fixes all the issues mentioned above:

• Use the list of tables in the workflow from MaterializeSettings
• We run select 1 from <table1> limit 1 for each table on each shard. This will give us both the existence of the table (check for table not found ) and a non-empty table.
• We can run these queries with READ UNCOMMITTED
• Run in parallel for each shard

For a workflow with 1000s of tables and 1000s of shards this will imply a large number of queries. We may want to add a --no-validations flag to bypass these checks in such cases. But since this is run during the Create step and with a weak isolation level, we may be fine without the flag for now. The flag might be a fail-safe though in case of some bug in this code, as pointed out above.

@noble, I suggest you wait before coding further on this until we reach an agreement on the final approach.

This approach avoids the issue of size of query/number of joined tables in a query that we have with the union approach. So that will also have to be batched and a query with several tables being joined even with a union may have performance impacts which might reduce the effect of the order of magnitude increase in queries.

[beingnoble03]

The changes were moved to: #16874. Closing this one.