Schema diff

Compare schemas writes a query into a new tab. It does not compare anything itself — you run the query, and the server does the comparing.

That is a deliberate design and the dialog says so out loud: Joinery does not compare them itself — the server does, when you run it. What you get back is a result set, which means it sorts, filters, copies and exports like any other — none of which a built-in diff view would give you.

Opening it

Where	What it pre-fills
A database’s right-click menu ▸ Compare Schemas…	That database as the source
⌘K ▸ Compare database schemas	The focused tab’s database as the source

Both refuse before opening if there is nothing to work with: with no connection you get Connect to a server before comparing schemas, and on a server with fewer than two databases loaded, … has only one database loaded — there is nothing to compare it to.

The comparison is always within one server. There is no cross-server compare.

The dialog

Two pickers — Source and Target — over the databases loaded for that server, and four checkboxes, all on by default:

• Tables and columns
• Views
• Stored procedures and functions
• Indexes

Generate comparison query is disabled until the form makes sense, and the reason sits beside the button rather than being left for you to guess:

Problem	What it says
A picker is empty	Pick a source and a target database.
Source and target are equal	Pick two different databases — a database always matches itself.
Every checkbox is off	Choose at least one thing to compare.

Pressing it opens a query tab pointed at the source database — that is where the query’s unqualified references resolve — and does not run it. Generating SQL rather than a diff is only worth anything if you can read it before it executes.

What the query looks like

A header comment naming both databases, then one banner-delimited section per box you ticked, in the order above. There is deliberately no timestamp in the header, so regenerating the same comparison produces byte-identical SQL — a saved comparison query does not show as modified just because you pressed the button again, and two runs can be diffed against each other.

Each section reports what one side has and the other does not, with a Location column naming which database that is. The tables section additionally reports column differences inside tables both sides have — a column missing on one side, or a type or length mismatch.

The SQL is per engine:

• SQL Server can name another database inside a query, so the comparison is one statement over three-part INFORMATION_SCHEMA names, plus sys.indexes for the index section. Index names are not unique per database in SQL Server, so the index comparison matches on schema and table as well as name.
• MySQL calls a database a schema and its information_schema spans all of them, so the same comparison becomes a self-join over two TABLE_SCHEMA values, with information_schema.STATISTICS in place of sys.indexes.

Database names are escaped on the way into the SQL — brackets doubled for identifiers, quotes doubled for literals — so a database called a]b or a'b still produces a statement that parses.

Note — the routines section covers functions as well as procedures, which is why the checkbox reads “Stored procedures and functions”.

PostgreSQL

PostgreSQL cannot query across databases, so there is no single statement to generate and Joinery does not pretend otherwise.

The menu item is still there on a PostgreSQL connection — hiding it would leave someone who read about the feature hunting for it — and the dialog opens, explains, and offers no Generate button:

PostgreSQL cannot query across databases, so there is no single statement that compares two of them. Connect to each database and compare the results, or install dblink / postgres_fdw.

Comparing two schemas inside one PostgreSQL database is a different feature and has not shipped.

Where this page's facts come from

Claim	Source
It generates a query and the server does the comparing	packages/renderer/src/features/schema-diff/diff-query.ts:1-8
The dialog’s title, description and primary-action wording	packages/renderer/src/features/schema-diff/schema-diff-dialog.tsx:81-86, 174-184
A database’s menu carries “Compare Schemas…”, on every engine	packages/renderer/src/shell/sidebar/node-menu.tsx:259-268
That entry pre-selects the clicked database as the source	packages/renderer/src/features/schema-diff/schema-diff-host.tsx:88-91
The palette entry “Compare database schemas” uses the focused tab’s database	packages/renderer/src/commands/catalogue.ts:675-686, schema-diff-host.tsx:81-86
The two pre-open refusals and their exact wording	packages/renderer/src/features/schema-diff/schema-diff-host.tsx:56-79
The comparison is within one server	packages/renderer/src/features/schema-diff/schema-diff-dialog.tsx:45-56, 96-99
Source and Target pickers over that server’s loaded databases	packages/renderer/src/features/schema-diff/schema-diff-dialog.tsx:105-135
The four sections, their labels and their default-on state	packages/renderer/src/features/schema-diff/diff-query.ts:40-56
The three disabled-reasons, shown beside the button	packages/renderer/src/features/schema-diff/diff-query.ts:87-99, schema-diff-dialog.tsx:72-76, 157-166
The tab opens on the source database and does not auto-execute	packages/renderer/src/features/schema-diff/schema-diff-host.tsx:102-111
Header comment, banners, and one section per ticked box in a fixed order	packages/renderer/src/features/schema-diff/diff-query.ts:101-122
No timestamp, so regeneration is byte-identical	packages/renderer/src/features/schema-diff/diff-query.ts:103-106
Each section reports one-side-only rows with a Location column	packages/renderer/src/features/schema-diff/diff-query.ts:148-166, 271-281
Column differences are reported for tables both sides have	packages/renderer/src/features/schema-diff/diff-query.ts:168-200, 283-312
SQL Server uses three-part names and sys.indexes	packages/renderer/src/features/schema-diff/diff-query.ts:19-21, 236-262
The index comparison matches on table and schema as well as index name	packages/renderer/src/features/schema-diff/diff-query.ts:143-146, 244-248
MySQL uses a TABLE_SCHEMA self-join and information_schema.STATISTICS	packages/renderer/src/features/schema-diff/diff-query.ts:265-268, 336-345
Identifiers and literals are escaped	packages/renderer/src/features/schema-diff/diff-query.ts:348-356
The routines section covers functions as well as procedures	packages/renderer/src/features/schema-diff/diff-query.ts:140-142, 218-234
PostgreSQL is refused, and the exact refusal sentence	packages/renderer/src/features/schema-diff/diff-query.ts:75-82, 96
The refusal renders inside the dialog with no Generate button	packages/renderer/src/features/schema-diff/schema-diff-dialog.tsx:89-102, 172-185
Comparing two schemas in one PostgreSQL database has not shipped	packages/renderer/src/features/schema-diff/diff-query.ts:24-27