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?_nocount=1 for opting out of table counts #1353

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?_nocount=1 for opting out of table counts#1353
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enhancementperformance
@simonw

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@simonw
simonw
opened on Jun 1, 2021

Running a trace against a CSV streaming export with the new _trace=1 feature from #1351 shows that the following code is executing a select count(*) from table for every page of results returned:

datasette/datasette/views/table.py

Lines 700 to 705 in d1d06ac

if count_sql and filtered_table_rows_count is None:
try:
count_rows = list(await db.execute(count_sql, from_sql_params))
filtered_table_rows_count = count_rows[0][0]
except QueryInterrupted:
pass

This is inefficient - a new ?_nocount=1 option would let us disable this count in the same way as #1349:

datasette/datasette/views/base.py

Lines 264 to 276 in d1d06ac

async def as_csv(self, request, database, hash, **kwargs):
stream = request.args.get("_stream")
# Do not calculate facets:
if not request.args.get("_nofacets"):
if not request.query_string:
new_query_string = "_nofacets=1"
else:
new_query_string = request.query_string + "&_nofacets=1"
new_scope = dict(
request.scope, query_string=new_query_string.encode("latin-1")
)
request.scope = new_scope
if stream:

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