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What happened in this migration
Damilare Agba describes moving a limited set of vendor accounts and related records from a production PostgreSQL database into a fresh database. Multiple services shared the source database, and a column named vendor_id did not consistently refer to the same ID space. The team reviewed the relevant entities and swept plausible ID columns for both profile and account IDs. Agba reports that the resulting map of relevant columns was complete for this project; that finding is specific to the migration, not a general guarantee that similarly named columns have consistent meanings.
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The team planned a dry run by performing the import inside a transaction and rolling it back. They expected PostgreSQL’s client to print a COPY n result for each table. Instead, the process exited with code 0 and printed no COPY results. Inspection revealed the key fact: import.sql contained no copy commands. The database had received no import work, so the transaction had nothing to undo. Agba’s incident account describes the mismatch between the expected output and the empty command file.
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Why a clean exit did not mean the dry run worked
A transaction-based dry run is an execution check: the import statements run against the target, and a rollback discards their changes. It is not a simulation that can prove work happened merely by starting and ending cleanly. If the generated input is empty, the client may complete without an error because it was not asked to execute an import.
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That distinction matters when evaluating a dry run. A zero exit status says the process did not report a failure; it does not establish that the intended commands were generated, that they ran, or that the expected records were loaded. In this incident, the absence of per-table COPY n output and the empty command file were stronger evidence than the exit code.
How zsh silently removed the import commands
The import file was generated with echo under zsh. The intended command lines began with copy, but zsh’s builtin echo interpreted the backslash-c sequence as a control escape: it stopped output at that point and suppressed the final newline. As a result, the generated file lacked the import commands.
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The zsh manual documents this behavior and recommends printf for portable text output. The correction in the incident was to use printf '%sn' rather than echo to write each line. See the zsh manual’s shell builtin documentation.
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- Inspect the generated file. Open the exact command file that will be passed to
psql. Check that it is nonempty and contains the expectedcopycommand for every intended table. In this case, seeing no commands inimport.sqlwould have exposed the problem before execution. - Look for operation-level evidence. For each expected table, compare the observed
COPY noutput with the expected work. No COPY lines should trigger investigation, even if the client exits with code 0. A count of zero may be valid for an intentionally empty input, but it should be explained by the input and the migration plan rather than inferred from a clean exit. - Confirm the target schema is ready. Agba reports that some target migrations were incomplete and had to be rerun. Before loading position-mapped CSV data, compare source and target column order and types, including enum types, so values are not loaded into an incompatible schema.
- Validate the committed import separately. Compare expected and actual row counts to catch obvious omissions or additions. Matching counts do not prove that the right values landed in the right rows. Agba also reports comparing full-row hashes and checking foreign-key-like links to validate content and relationships.
Watch for shell-specific script failures
The same migration account describes a separate loop bug: a table-list variable was unquoted, and the loop behaved differently in bash and zsh. In bash, the list split into the expected items; in zsh, it was treated as one filename, producing a visible error. That failure was noisy, unlike the silent no-op from the missing import commands.
Do not assume a shell script behaves the same in every shell. Use a deliberate data structure and quoting strategy for table names, and run the generator in the shell that will execute it. For literal output, the zsh manual’s recommendation to use printf avoids echo‘s shell-dependent escape handling.
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