fix(migrator): support functional prefix indexes for PostgreSQL

[hamid17amu]

• Do only one thing
• Non breaking API changes
• Tested

What did this pull request do?

1. The Problem

GORM currently uses MySQL-specific syntax column(length) when generating indexes with a length tag. While this works for MySQL, it is invalid in PostgreSQL. Currently, when a user defines gorm:"index:idx_name,length:10" in a Postgres environment, GORM generates:
CREATE INDEX "idx_name" ON "table" ("column"(10))
This results in either a syntax error or a standard B-tree index that ignores the length, depending on the driver version.

2. The Solution

This PR introduces two changes:

Schema Parser: Updates parseFieldIndexes to be case-insensitive when looking for the length tag, ensuring length:10 and LENGTH:10 both populate theIndexOption.Lengthfield.

Migrator: Updates BuildIndexOptions to check the database dialect. If the dialect is PostgreSQL, it uses the functional LEFT(column, length) syntax wrapped in parentheses.

3. Impact

• MySQL/SQLite: No change (standard syntax preserved).
• PostgreSQL: Now correctly creates prefix indexes as functional indexes.

User Case Description

1. Define a model with a prefix index:

type User struct {
    Name string `gorm:"index:idx_name,length:10"`
}

2. Run AutoMigrate on a PostgreSQL database.
3. Verify the index using \d users.
4. Expected result: idx_name btree (left(name, 10))

Resolves #7752

[propel-code-bot]

Fix BuildIndexOptions to generate PostgreSQL-compatible prefix index expressions

This PR updates index SQL generation in migrator/migrator.go so prefix index handling is dialect-aware. In Migrator.BuildIndexOptions, when opt.Length > 0, PostgreSQL now uses LEFT(column, length) expression syntax, while non-PostgreSQL dialects retain the previous column(length) format.

The change appears to be a targeted bug fix for invalid PostgreSQL index SQL generation from length index tags, while preserving existing behavior for MySQL/SQLite. The final patch also addresses earlier review feedback that warned against applying PostgreSQL functional syntax globally.

Key Changes

• Modified Migrator.BuildIndexOptions in migrator/migrator.go to branch behavior by detected dialect.
• Added dialect detection via m.DB.Dialector type string check and strings.Contains(..., "postgres").
• For PostgreSQL, length-based index options now emit LEFT(%s,%d) instead of appending (%d) directly to a quoted column.
• For other dialects, preserved existing str += fmt.Sprintf("(%d)", opt.Length) behavior.

Affected Areas

• migrator/migrator.go
• Migrator.BuildIndexOptions index option SQL construction path
• Auto-migration index creation paths that rely on BuildIndexOptions

This summary was automatically generated by @propel-code-bot

[propel-code-bot]

Review found no issues; the PostgreSQL prefix-index support change appears safe and well-scoped.

Status: No Issues Found | Risk: Low

Review Details

📁 1 files reviewed | 💬 0 comments

[hamid17amu]

@jinzhu Please review