Add the document to backfill using tx_meta

backfill.md

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+# How to Backfill history_transactions table using tx_meta (xdr)
+
+In cases of new column requests/addressing bugs, airflow backfill jobs are needed to be kicked off. This process is both time and money consuming.
+
+This document outlies methods to extract required field from transaction envelope (tx_meta). This documents takes an example of `fee_account_muxed` field. However, it can be extended to other fields as well.
+
+We will use [js-stellar-base](https://github.com/stellar/js-stellar-base) library to parse the XDRs and then use Javascript UDF in Bigquery to apply the transformation to the dataset.
+
+Referred medium article: [Using NPM Library in Google BigQuery UDF](https://medium.com/analytics-vidhya/using-npm-library-in-google-bigquery-udf-8aef01b868f4)
+
+# Setting up JS UDF in Bigquery
+
+```
+git clone https://github.com/stellar/js-stellar-base.git
+cd js-stellar-base
+yarn
+npx webpack --config config/webpack.config.browser.js
+```
+
+Above will create following files in `js-stellar-base/dist/` directory:
+- stellar-base.js
+- stellar-base.min.js
+
+Copy above files in a GCS bucket.
+
+# Writing JS Function
+
+Following is example to extract FeeAccountMuxed:
+
+```
+    let tx_meta = "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";
+
+    let txe = StellarBase.xdr.TransactionEnvelope.fromXDR(tx_meta, 'base64');
+    let tx = txe.feeBump();
+    let sourceAccount = StellarBase.encodeMuxedAccountToAddress(tx.tx().feeSource());
+    console.log(sourceAccount)
+```
+
+Above will print `MBX64B6V7KV2KVHJ66IIKLNEIKCHF5BI57BNMF3U6RJIUHNBMXSJEAAACYGGCDRL6UFO2`, which is the `fee_account_muxed` value
+
+# Wrapping JS function as UDF
+
+```
+CREATE TEMP FUNCTION getFeeBumpAccountIfExists(tx_meta STRING)
+RETURNS STRING
+LANGUAGE js
+OPTIONS (
+  library=["gs://stellar-test-js-udf/stellar-base.min.js"] -- path to js library in GCS
+)
+AS r"""
+    let txe = StellarBase.xdr.TransactionEnvelope.fromXDR(tx_meta, 'base64');
+    let tx = txe.feeBump();
+    let sourceAccount = StellarBase.encodeMuxedAccountToAddress(tx.tx().feeSource());
+    return sourceAccount
+
+""";
+
+WITH fee_bump_transactions AS
+  (  
+    SELECT batch_run_date, transaction_hash, tx_envelope as tx_meta FROM `test_crypto_stellar.history_transactions`
+    WHERE
+    batch_run_date BETWEEN DATETIME("2024-07-01") AND DATETIME_ADD("2024-07-20", INTERVAL 1 MONTH) 
+    and inner_transaction_hash is not null -- filter in fee bump transactions
+  ),
+  calculated_fee_account as (
+    SELECT batch_run_date, transaction_hash, getFeeBumpAccountIfExists(tx_meta) as fee_account
+    FROM fee_bump_transactions
+  ),
+  calculated_fee_muxed_account as (
+    SELECT batch_run_date, transaction_hash, fee_account FROM calculated_fee_account
+    WHERE fee_account LIKE 'M%' -- muxed accounts
+  )
+  SELECT batch_run_date, transaction_hash, fee_account as fee_account_muxed FROM calculated_fee_muxed_account
+```
+
+Sample output for above JS UDF.
+Row	| transaction_hash | fee_account
+-- | -- | -- 
+1	| f5f5b0aaf758896ef8c5b4807f41c77d15c11977eecf2b0e4769d777324a2d11 |MCBD54KAHHA4AK4DOZWOSX5O5OZ4OI54N24QITDSFLPD7EG2WY2AMAAACYGGCDRL6UBUA
+2	| a9e49dff6202663633b83f3645fbf8c2cfeb915db99b2b884a86791b9f8eae2f | MBX64B6V7KV2KVHJ66IIKLNEIKCHF5BI57BNMF3U6RJIUHNBMXSJEAAACYGGCDRL6UFO2
+3	| 00dba50c8689477e6990103338a0eb326725e07a7b7ff187359abf11c23c582a| MC5BEU3DCIMHOHRQDVDAPEPZGMBBALPJ3IQY23VTXC3454SQMNWVSAAACYGGCDRL6UX42
+4	| 2e1c53a9fe1d48ddc493febe467178994e669e3eebf3a4cca646b3cb666616de|MAMYAUW45TC54C3QORQP7OOFYKOXCJTXOG2WIV5LP2HDMR67MWP6IAAACYGGCDRL6VCZM