我正在使用pandas,sqlite和sqlalchemy搜索一串字符串以获取子字符串。这个项目的灵感来自于this tutorial.Pandas + SQLite“无法使用索引”错误
首先,我创建了一个字符串列的sqlite数据库。然后我遍历一个单独的字符串文件并在数据库中搜索这些字符串。
我发现这个过程很慢,所以我做了一些研究,发现我需要在我的专栏上建立一个索引。当我按照sqlite shell中提供的here的说明操作时,一切似乎都正常。
但是,当我尝试在我的python脚本中创建索引时,出现“无法使用索引”错误。
import pandas as pd
from sqlalchemy import create_engine # database connection
import datetime as dt
def load_kmer_db(disk_engine, chunk_size, encoding='utf-8'):
start = dt.datetime.now()
j = 0
index_start = 1
for df in pd.read_csv('fake.kmers.csv', chunksize=chunk_size, iterator=True, encoding=encoding):
df.index += index_start
j += 1
df.to_sql('data', disk_engine.raw_connection(), if_exists='append', index=True, index_label='kmer_index')
index_start = df.index[-1] + 1
def search_db_for_subsequence(disk_engine, sequence):
"""
:param disk_engine: Disk engine for database containing query sequences
:param sequence: Sequence for finding subsequences in the database
:return: A data frame with the subsequences of sequence
"""
return pd.read_sql_query("SELECT kmer FROM data INDEXED BY kmer_index WHERE '" + sequence + "' LIKE '%' || kmer || '%'", disk_engine)
if __name__ == "__main__":
import argparse
parser = argparse.ArgumentParser()
parser.add_argument('kmers', type=str, metavar='<kmer_file.txt>', help='text file with kmers')
parser.add_argument('reads', type=str, metavar='<reads.fastq>', help='Reads to filter by input kmers')
# Get the command line arguments.
args = parser.parse_args()
kmer_file = args.kmers
reads_file = args.reads
# Initialize database with filename 311_8M.db
disk_engine = create_engine('sqlite:///311_8M.db') # This requires ipython to be installed
load_kmer_db(disk_engine, 200)
#****** Try explicitly calling the create index command
#****** using the sqlite module.
import sqlite3
conn = sqlite3.connect('311_8M.db')
c = conn.cursor()
c.execute("CREATE INDEX kmer_index ON data(kmer);")
reads = SeqReader(reads_file)
for read in reads.parse_fastq():
count += 1
sequence = read[1]
df = search_db_for_subsequence(
disk_engine,
sequence
)
可以看到我首先尝试通过将正确的关键字参数传递给to_sql方法来创建索引。当我这样做时,我得到一个错误,指出索引找不到。然后我通过sqlite3模块明确地创建了索引,该模块产生了“无法使用索引”错误。
因此,现在看来我已经创建了索引,但由于某种原因,我无法使用它。为什么会这样?而且,如何使用pandas api创建索引,而不必使用sqlite3模块?
错误消息“无法使用索引”似乎与“pd.read_sql_query()”调用有关,而不是您直接使用sqlite3模块创建索引的部分。 – bernie
是的,它似乎是我成功创建索引,那为什么我无法使用它? – Malonge
我认为这与你使用LIKE'%[某个词]%' – bernie