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CrateDB: Hello, World!
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‎README-CRATEDB.md‎

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# CrateDB Model Context Protocol Server
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## About
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This CrateDB MCP Server is based on the PostgreSQL Model Context Protocol (PG-MCP) Server.
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[pg-mcp] uses [asyncpg], so it can also be used with CrateDB.
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`server/resources/schema.py` received a few adjustments to compensate for
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missing metadata features of CrateDB, nothing serious.
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## Usage
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Start CrateDB.
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```shell
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docker run --rm \
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--name=cratedb --publish=4200:4200 --publish=5432:5432 \
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--env=CRATE_HEAP_SIZE=2g crate/crate:nightly \
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-Cdiscovery.type=single-node
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```
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Initialize Python environment.
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```shell
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git clone https://github.com/crate-workbench/pg-mcp --branch=cratedb
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cd pg-mcp
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uv venv --python 3.13 --seed .venv
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uv sync --frozen
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```
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Run MCP server and test program.
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```shell
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uv run -m server.app
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uv run test.py "postgresql://crate@localhost/doc"
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```
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Run example Claude session (untested).
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```shell
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export DATABASE_URL=postgresql://crate@localhost
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export ANTHROPIC_API_KEY=...
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uv run -m client.claude_cli "Give me 5 Austria mountains (querying specific tables, like sys.summits)"
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```
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[asyncpg]: https://pypi.org/project/asyncpg
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[pg-mcp]: https://github.com/stuzero/pg-mcp

‎server/resources/schema.py‎

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Original file line numberDiff line numberDiff line change
@@ -18,8 +18,7 @@ async def db_info(conn_id: str):
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# Get all non-system schemas
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schemas_query = """
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SELECT
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schema_name,
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obj_description(pg_namespace.oid) as description
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schema_name
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FROM information_schema.schemata
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JOIN pg_namespace ON pg_namespace.nspname = schema_name
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WHERE
@@ -45,9 +44,7 @@ async def db_info(conn_id: str):
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# Get all tables in the schema
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tables_query = """
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SELECT
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t.table_name,
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obj_description(format('"%s"."%s"', t.table_schema, t.table_name)::regclass::oid) as description,
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pg_stat_get_tuples_inserted(format('"%s"."%s"', t.table_schema, t.table_name)::regclass::oid) as row_count
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t.table_name
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FROM information_schema.tables t
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WHERE
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t.table_schema = $1
@@ -76,16 +73,15 @@ async def db_info(conn_id: str):
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c.column_name,
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c.data_type,
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c.is_nullable,
79-
c.column_default,
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col_description(format('"%s"."%s"', c.table_schema, c.table_name)::regclass::oid, c.ordinal_position) as description
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c.column_default
8177
FROM information_schema.columns c
8278
WHERE
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c.table_schema = $1 AND
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c.table_name = $2
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ORDER BY c.ordinal_position
8682
"""
8783
columns = await execute_query(columns_query, conn_id, [schema_name, table_name])
88-
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# Get constraints for this table to identify primary keys, etc.
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constraints_query = """
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SELECT
@@ -97,18 +93,15 @@ async def db_info(conn_id: str):
9793
WHEN c.contype = 'f' THEN 'FOREIGN KEY'
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WHEN c.contype = 'c' THEN 'CHECK'
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ELSE 'OTHER'
100-
END as constraint_type_desc,
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ARRAY_AGG(col.attname ORDER BY u.attposition) as column_names
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END as constraint_type_desc
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FROM
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pg_constraint c
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JOIN
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pg_namespace n ON n.oid = c.connamespace
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JOIN
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pg_class t ON t.oid = c.conrelid
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LEFT JOIN
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LATERAL unnest(c.conkey) WITH ORDINALITY AS u(attnum, attposition) ON TRUE
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LEFT JOIN
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pg_attribute col ON col.attrelid = t.oid AND col.attnum = u.attnum
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pg_attribute col ON col.attrelid = t.oid
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WHERE
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n.nspname = $1
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AND t.relname = $2
@@ -118,7 +111,7 @@ async def db_info(conn_id: str):
118111
c.contype, c.conname
119112
"""
120113
constraints = await execute_query(constraints_query, conn_id, [schema_name, table_name])
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# Process columns and add constraint information
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for column in columns:
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column_name = column['column_name']
@@ -128,56 +121,22 @@ async def db_info(conn_id: str):
128121
for constraint in constraints:
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if column_name in constraint.get('column_names', []):
130123
column_constraints.append(constraint['constraint_type_desc'])
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# Add column info
133126
column_info = {
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"name": column_name,
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"type": column['data_type'],
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"nullable": column['is_nullable'] == 'YES',
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"default": column['column_default'],
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"description": column['description'],
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"constraints": column_constraints
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}
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table_info["columns"].append(column_info)
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# Process foreign key constraints
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foreign_keys_query = """
146-
SELECT
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c.conname as constraint_name,
148-
ARRAY_AGG(col.attname ORDER BY u.attposition) as column_names,
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nr.nspname as referenced_schema,
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ref_table.relname as referenced_table,
151-
ARRAY_AGG(ref_col.attname ORDER BY u2.attposition) as referenced_columns
152-
FROM
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pg_constraint c
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JOIN
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pg_namespace n ON n.oid = c.connamespace
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JOIN
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pg_class t ON t.oid = c.conrelid
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JOIN
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pg_class ref_table ON ref_table.oid = c.confrelid
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JOIN
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pg_namespace nr ON nr.oid = ref_table.relnamespace
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LEFT JOIN
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LATERAL unnest(c.conkey) WITH ORDINALITY AS u(attnum, attposition) ON TRUE
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LEFT JOIN
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pg_attribute col ON col.attrelid = t.oid AND col.attnum = u.attnum
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LEFT JOIN
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LATERAL unnest(c.confkey) WITH ORDINALITY AS u2(attnum, attposition) ON TRUE
168-
LEFT JOIN
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pg_attribute ref_col ON ref_col.attrelid = c.confrelid AND ref_col.attnum = u2.attnum
170-
WHERE
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n.nspname = $1
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AND t.relname = $2
173-
AND c.contype = 'f'
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GROUP BY
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c.conname, nr.nspname, ref_table.relname
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ORDER BY
177-
c.conname
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"""
179-
foreign_keys = await execute_query(foreign_keys_query, conn_id, [schema_name, table_name])
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137+
# CrateDB does not provide foreign key constraints.
138+
foreign_keys = []
139+
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for fk in foreign_keys:
182141
fk_info = {
183142
"name": fk['constraint_name'],
@@ -201,8 +160,7 @@ async def list_schemas(conn_id: str):
201160
"""List all non-system schemas in the database."""
202161
query = """
203162
SELECT
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schema_name,
205-
obj_description(pg_namespace.oid) as description
163+
schema_name
206164
FROM information_schema.schemata
207165
JOIN pg_namespace ON pg_namespace.nspname = schema_name
208166
WHERE
@@ -217,9 +175,7 @@ async def list_schema_tables(conn_id: str, schema: str):
217175
"""List all tables in a specific schema with their descriptions."""
218176
query = """
219177
SELECT
220-
t.table_name,
221-
obj_description(format('"%s"."%s"', t.table_schema, t.table_name)::regclass::oid) as description,
222-
pg_stat_get_tuples_inserted(format('"%s"."%s"', t.table_schema, t.table_name)::regclass::oid) as total_rows
178+
t.table_name
223179
FROM information_schema.tables t
224180
WHERE
225181
t.table_schema = $1
@@ -236,8 +192,7 @@ async def get_table_columns(conn_id: str, schema: str, table: str):
236192
c.column_name,
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c.data_type,
238194
c.is_nullable,
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c.column_default,
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col_description(format('"%s"."%s"', c.table_schema, c.table_name)::regclass::oid, c.ordinal_position) as description
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c.column_default
241196
FROM information_schema.columns c
242197
WHERE
243198
c.table_schema = $1 AND
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252207
query = """
253208
SELECT
254209
i.relname as index_name,
255-
pg_get_indexdef(i.oid) as index_definition,
256-
obj_description(i.oid) as description,
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am.amname as index_type,
258-
ARRAY_AGG(a.attname ORDER BY k.i) as column_names,
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ix.indisunique as is_unique,
260212
ix.indisprimary as is_primary,
261213
ix.indisexclusion as is_exclusion
@@ -270,9 +222,7 @@ async def get_table_indexes(conn_id: str, schema: str, table: str):
270222
JOIN
271223
pg_am am ON i.relam = am.oid
272224
LEFT JOIN
273-
LATERAL unnest(ix.indkey) WITH ORDINALITY AS k(attnum, i) ON TRUE
274-
LEFT JOIN
275-
pg_attribute a ON a.attrelid = t.oid AND a.attnum = k.attnum
225+
pg_attribute a ON a.attrelid = t.oid
276226
WHERE
277227
n.nspname = $1
278228
AND t.relname = $2
@@ -299,14 +249,11 @@ async def get_table_constraints(conn_id: str, schema: str, table: str):
299249
WHEN c.contype = 'x' THEN 'EXCLUSION'
300250
ELSE 'OTHER'
301251
END as constraint_type_desc,
302-
obj_description(c.oid) as description,
303-
pg_get_constraintdef(c.oid) as definition,
304252
CASE
305253
WHEN c.contype = 'f' THEN
306254
(SELECT nspname FROM pg_namespace WHERE oid = ref_table.relnamespace) || '.' || ref_table.relname
307255
ELSE NULL
308-
END as referenced_table,
309-
ARRAY_AGG(col.attname ORDER BY u.attposition) as column_names
256+
END as referenced_table
310257
FROM
311258
pg_constraint c
312259
JOIN
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316263
LEFT JOIN
317264
pg_class ref_table ON ref_table.oid = c.confrelid
318265
LEFT JOIN
319-
LATERAL unnest(c.conkey) WITH ORDINALITY AS u(attnum, attposition) ON TRUE
320-
LEFT JOIN
321-
pg_attribute col ON col.attrelid = t.oid AND col.attnum = u.attnum
266+
pg_attribute col ON col.attrelid = t.oid
322267
WHERE
323268
n.nspname = $1
324269
AND t.relname = $2
@@ -335,8 +280,6 @@ async def get_index_details(conn_id: str, schema: str, table: str, index: str):
335280
query = """
336281
SELECT
337282
i.relname as index_name,
338-
pg_get_indexdef(i.oid) as index_definition,
339-
obj_description(i.oid) as description,
340283
am.amname as index_type,
341284
ix.indisunique as is_unique,
342285
ix.indisprimary as is_primary,
@@ -345,9 +288,7 @@ async def get_index_details(conn_id: str, schema: str, table: str, index: str):
345288
ix.indisclustered as is_clustered,
346289
ix.indisvalid as is_valid,
347290
i.relpages as pages,
348-
i.reltuples as rows,
349-
ARRAY_AGG(a.attname ORDER BY k.i) as column_names,
350-
ARRAY_AGG(pg_get_indexdef(i.oid, k.i, false) ORDER BY k.i) as column_expressions
291+
i.reltuples as rows
351292
FROM
352293
pg_index ix
353294
JOIN
@@ -359,9 +300,7 @@ async def get_index_details(conn_id: str, schema: str, table: str, index: str):
359300
JOIN
360301
pg_am am ON i.relam = am.oid
361302
LEFT JOIN
362-
LATERAL unnest(ix.indkey) WITH ORDINALITY AS k(attnum, i) ON TRUE
363-
LEFT JOIN
364-
pg_attribute a ON a.attrelid = t.oid AND a.attnum = k.attnum
303+
pg_attribute a ON a.attrelid = t.oid
365304
WHERE
366305
n.nspname = $1
367306
AND t.relname = $2
@@ -389,17 +328,14 @@ async def get_constraint_details(conn_id: str, schema: str, table: str, constrai
389328
WHEN c.contype = 'x' THEN 'EXCLUSION'
390329
ELSE 'OTHER'
391330
END as constraint_type_desc,
392-
obj_description(c.oid) as description,
393-
pg_get_constraintdef(c.oid) as definition,
394331
CASE
395332
WHEN c.contype = 'f' THEN
396333
(SELECT nspname FROM pg_namespace WHERE oid = ref_table.relnamespace) || '.' || ref_table.relname
397334
ELSE NULL
398335
END as referenced_table,
399-
ARRAY_AGG(col.attname ORDER BY u.attposition) as column_names,
400336
CASE
401337
WHEN c.contype = 'f' THEN
402-
ARRAY_AGG(ref_col.attname ORDER BY u2.attposition)
338+
ARRAY_AGG(ref_col.attname)
403339
ELSE NULL
404340
END as referenced_columns
405341
FROM
@@ -411,13 +347,9 @@ async def get_constraint_details(conn_id: str, schema: str, table: str, constrai
411347
LEFT JOIN
412348
pg_class ref_table ON ref_table.oid = c.confrelid
413349
LEFT JOIN
414-
LATERAL unnest(c.conkey) WITH ORDINALITY AS u(attnum, attposition) ON TRUE
415-
LEFT JOIN
416-
pg_attribute col ON col.attrelid = t.oid AND col.attnum = u.attnum
417-
LEFT JOIN
418-
LATERAL unnest(c.confkey) WITH ORDINALITY AS u2(attnum, attposition) ON c.contype = 'f'
350+
pg_attribute col ON col.attrelid = t.oid
419351
LEFT JOIN
420-
pg_attribute ref_col ON c.contype = 'f' AND ref_col.attrelid = c.confrelid AND ref_col.attnum = u2.attnum
352+
pg_attribute ref_col ON c.contype = 'f' AND ref_col.attrelid = c.confrelid
421353
WHERE
422354
n.nspname = $1
423355
AND t.relname = $2

‎server/tools/query.py‎

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@@ -78,7 +78,7 @@ async def pg_explain(query: str, conn_id: str, params=None):
7878
Complete JSON-formatted execution plan
7979
"""
8080
# Prepend EXPLAIN to the query
81-
explain_query = f"EXPLAIN (FORMAT JSON) {query}"
81+
explain_query = f"EXPLAIN {query}"
8282

8383
# Execute the explain query
8484
result = await execute_query(explain_query, conn_id, params)

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